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The information in this prospectus supplement and the accompanying prospectus is not complete and may be changed. This prospectus supplement and the accompanying prospectus are not an offer to sell these securities and are not soliciting an offer to buy these securities in any state where the offer or sale is not permitted.

Filed Pursuant to Rule 424(b)(5)

Registration No. 333-282881

Subject to completion, dated March 2, 2026

Preliminary prospectus supplement
(To Prospectus dated October 29, 2024)

$550,000,000

Common stock

We are offering $150,000,000 of shares of our common stock, par value $0.001 per share in this offering. In addition, the forward sellers referred to below are offering $400,000,000 of shares of our common stock. We expect to enter into forward sale agreements with each of JPMorgan Chase Bank, National Association, and Wells Fargo Bank, National Association, which are referred to as the “forward purchasers,” with respect to an aggregate of $400,000,000 of shares of our common stock. In connection with these forward sale agreements, the forward purchasers or their affiliates and/or agents, which are referred to in such capacity as the “forward sellers,” at our request, are borrowing from third parties and selling to the underwriters an aggregate of $400,000,000 of shares of our common stock. If in the good faith, commercially reasonable judgment of a forward purchaser, it or its affiliate is unable to borrow and deliver for sale on the anticipated closing date, a number of shares of our common stock underlying its forward sale agreement, or it or its affiliate would be unable to borrow, at a stock loan rate not greater than a specified rate, and deliver for sale on the anticipated closing date such number of shares of our common stock, or if certain other conditions to such forward seller’s obligations have not been satisfied, then we will issue and sell directly to the underwriters a number of shares of our common stock equal to the number of shares that such forward seller does not borrow and deliver.

We expect to receive proceeds from the sale of $150,000,000 of shares of common stock sold by us in this offering, but we will not initially receive any proceeds from the sale of the shares of our common stock sold by the forward sellers to the underwriters, except in certain circumstances described in this prospectus supplement, including the last sentence of the previous paragraph. Each forward sale agreement provides for settlement on a settlement date or dates to be specified at our discretion on or prior to March , 2028. If we elect to cash settle all or a portion of the forward sale agreements, we may not receive any proceeds from such election, and we may owe cash to one or more of the forward purchasers. If we elect to net share settle all or a portion of the forward sale agreements, we will not receive any cash proceeds from such election, and we may owe shares of our common stock to one or more of the forward purchasers. See “Underwriting (Conflicts of Interest) — Forward Sale Agreements” for a description of the forward sale agreements.

We intend to use the net proceeds from this offering, together with the Debt Financing (as defined herein), to finance our proposed acquisition of Quadvest, L.P. and Quadvest Wholesale, LLC, or the “Quadvest Acquisition,” and to pay related fees and expenses. This offering is not conditioned on the consummation of the Quadvest Acquisition or the Debt Financing. The Quadvest Acquisition, if completed, will occur subsequent to the closing of this offering. If for any reason the Quadvest Acquisition does not close, then we expect to use the net proceeds from this offering for general corporate purposes, which may include acquisitions, capital expenditures, share repurchases or debt repayment, and we will not have any obligation to repurchase any or all of the shares of our common stock sold in this offering. See “Summary — The Quadvest Acquisition” and “Use of Proceeds.”

Our common stock is listed on the Nasdaq Global Select Market, or “Nasdaq,” under the symbol “HTO”. On February 27, 2026, the closing price for our common stock was $53.79 per share.

Per share

Total

Public offering price

Underwriting discounts and commissions

Proceeds, before expenses, to us(1)

(1)

We expect to receive estimated net proceeds from the sale of shares of our common stock, before expenses, of approximately $ (or approximately $ if the underwriters’ option to purchase additional shares of our common stock is exercised in full, as described in detail below). We expect to receive net proceeds, before expenses, from the sale of our common stock of approximately $ upon full physical settlement of the forward sale agreements, which we expect to occur on or prior to March , 2028. For purposes of calculating the estimated net proceeds to us, we have assumed that the forward sale agreements are fully physically settled based on the initial forward sale price of $ per share, which is equal to the price to the public per share less the underwriting discount shown above. The forward sale price is subject to adjustment pursuant to the forward sale agreements, and the actual proceeds, if any, will be calculated as described in this prospectus supplement. Although we expect to settle the forward sale agreements entirely by the full physical delivery of shares of our common stock to the forward purchasers in exchange for cash proceeds, we may, except in certain circumstances, elect cash settlement or net share settlement for all or a portion of our obligations under one or more forward sale agreements, in which case we may receive no cash proceeds or substantially less cash proceeds than is reflected in the above table upon settlement, or we may be required to deliver cash or shares of our common stock to the forward purchasers. See “Underwriting — Forward Sale Agreements” for a description of the forward sale agreements.

The underwriters have advised us and the forward sellers that they initially propose to offer the shares of our common stock to the public at the public offering price that appears on the cover page of this prospectus supplement and may offer shares to selected dealers at the public offering price minus a concession of up to $ per share. After the initial offering, the underwriters may change the public offering price and any other selling terms. The underwriters may offer and sell shares through certain of their affiliates.

We have granted the underwriters an option to purchase up to an additional $82,500,000 of shares of our common stock at a price of $ per share for a period of 30 days from the date of this prospectus supplement.

Investing in our common stock involves risks. See “Risk Factors” beginning on page S-10 to read about factors you should consider before buying shares of our common stock.

Neither the Securities and Exchange Commission, or the SEC, nor any state securities commission has approved or disapproved of these securities, or determined if this prospectus supplement and the accompanying prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

The underwriters expect to deliver the shares of our common stock against payment on or about March , 2026.

Joint book-running managers

J.P. Morgan

Wells Fargo Securities

Prospectus supplement dated March , 2026

TABLE OF CONTENTS

Prospectus supplement

	Page
CAUTIONARY STATEMENT CONCERNING FORWARD-LOOKING STATEMENTS		S-v
SUMMARY		S-1
THE OFFERING		S-6
RISK FACTORS		S-10
USE OF PROCEEDS		S-14
CAPITALIZATION		S-16
DIVIDEND POLICY		S-17
UNAUDITED PRO FORMA CONDENSED COMBINED FINANCIAL INFORMATION		S-18
UNITED STATES FEDERAL TAX CONSIDERATIONS TO NON-U.S. HOLDERS		S-30
UNDERWRITING (CONFLICTS OF INTEREST)		S-34
VALIDITY OF THE SHARES		S-44
EXPERTS		S-44

Prospectus

	Page
ABOUT THIS PROSPECTUS		1
CAUTIONARY NOTE REGARDING FORWARD-LOOKING STATEMENTS		2
THE COMPANY		3
RISK FACTORS		4
USE OF PROCEEDS		5
DESCRIPTION OF DEBT SECURITIES		6
DESCRIPTION OF CAPITAL STOCK		13
DESCRIPTION OF DEPOSITARY SHARES		16
DESCRIPTION OF WARRANTS		20
DESCRIPTION OF UNITS		23
SELLING SECURITYHOLDERS		24
PLAN OF DISTRIBUTION		25
VALIDITY OF COMMON STOCK		27
EXPERTS		28
INCORPORATION OF DOCUMENTS BY REFERENCE		29
WHERE YOU CAN FIND MORE INFORMATION		30

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You should rely only on the information contained in or incorporated by reference into this prospectus supplement, the accompanying prospectus or any related free writing prospectus prepared by or on behalf of us. Neither we nor any of the underwriters has authorized anyone to provide you with different or additional information. If anyone provides you with different or additional information, you should not rely on it. The information in this prospectus supplement, the accompanying prospectus or any free writing prospectus and the documents incorporated by reference herein and therein is accurate only as of the respective dates of such documents. Our business, financial condition, results of operations and prospects may have changed since those dates.

The distribution of this prospectus supplement and the accompanying prospectus and the offering of the shares of common stock in certain jurisdictions may be restricted by law. Persons into whose possession this prospectus supplement and the accompanying prospectus come should inform themselves about and observe any such restrictions. This prospectus supplement and the accompanying prospectus do not constitute, and may not be used in connection with, an offer or solicitation by anyone in any jurisdiction in which such offer or solicitation is not authorized or in which the person making such offer or solicitation is not qualified to do so or to any person to whom it is unlawful to make such offer or solicitation.

ABOUT THIS PROSPECTUS SUPPLEMENT

This document is in two parts. The first part is this prospectus supplement, including the documents incorporated by reference, which describes the specific terms of the common stock we are currently offering. The second part, the accompanying prospectus, gives more general information, some of which may not apply to the common stock we are currently offering in this prospectus supplement. See “Description of Capital Stock” in the accompanying prospectus. In the event that information in this prospectus supplement is inconsistent with information in the accompanying prospectus, you should rely on the information in this prospectus supplement.

You should not assume that the information contained in this prospectus supplement and the accompanying prospectus is accurate on any date subsequent to the date set forth on the front of the document or that any information we have incorporated by reference is correct on any date subsequent to the date of the document incorporated by reference, even though this prospectus supplement and the accompanying prospectus are delivered on a later date. Information that we file with the Securities and Exchange Commission (the “SEC”) subsequent to the date on the cover of this prospectus supplement, and prior to the completion of this offering, will automatically update and supersede the information contained in this prospectus supplement and the accompanying prospectus. See “Where You Can Find More Information” and “Incorporation of Documents by Reference.”

The securities are being offered for sale only in jurisdictions where it is lawful to make such offers. The distribution of this prospectus supplement and the accompanying prospectus and the offering of the securities in other jurisdictions may also be restricted by law. Persons who receive this prospectus supplement and the accompanying prospectus should inform themselves about and observe any such restrictions. This prospectus supplement and the accompanying prospectus do not constitute, and may not be used in connection with, an offer or solicitation by anyone in any jurisdiction in which such offer or solicitation is not authorized, in which the person making such offer or solicitation is not authorized or in which the person making such offer or solicitation is not qualified to do so or to any person to whom it is unlawful to make such offer or solicitation. See “Underwriting (Conflicts of Interest)” in this prospectus supplement.

This prospectus supplement and the accompanying prospectus are part of an automatic shelf registration statement that we have filed with the SEC as a “well-known seasoned issuer” as defined in Rule 405 under the Securities Act. Under this shelf registration process, we may sell, at any time and from time to time, in one or more offerings, any of the securities described in the accompanying prospectus, including the common stock, up to an indeterminate amount, of which this offering is a part. In this prospectus supplement, we provide you with specific information about the terms of the common stock and this offering.

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Unless the context otherwise requires (including on the cover page and in “Summary — The Offering”) or as otherwise stated in “Risk Factors,” the terms the “Company,” “we,” “us,” “our” or similar terms and “H2O America” refers to H2O America, together with its consolidated subsidiaries, without giving effect to the Quadvest Acquisition, “Quadvest” refers collectively to Quadvest L.P. and Quadvest Wholesale, LLC, together with their consolidated subsidiaries, without giving effect to the Quadvest Acquisition, and “combined company” refers to us, after giving effect to the Quadvest Acquisition.

WHERE YOU CAN FIND MORE INFORMATION

This prospectus supplement is part of a registration statement on Form S-3 that we have filed with the SEC. This prospectus supplement, filed as part of the registration statement, does not contain all the information set forth in the registration statement and its exhibits and schedules, portions of which have been omitted as permitted by the rules and regulations of the SEC. For further information about us, we refer you to the registration statement and to its exhibits and schedules. Certain information in the registration statement has been omitted from this prospectus supplement in accordance with the rules of the SEC.

We are subject to the reporting and information requirements of the Exchange Act and, in accordance therewith, file annual, quarterly and special reports, proxy statements and other information with the SEC. These documents may be accessed through the SEC’s electronic data gathering, analysis and retrieval system, or EDGAR, via electronic means, including the SEC’s home page on the Internet (www.sec.gov). Written requests for such copies should be directed to H2O America, 110 West Taylor Street, San Jose, CA 95110; telephone: (408) 279-7800, and our website is located at www.h2o-america.com. Information contained on our website is not incorporated by reference into this prospectus supplement and, therefore, is not part of this prospectus supplement or any accompanying prospectus.

We have the authority to designate and issue more than one class or series of stock having various preferences, conversion and other rights, voting powers, restrictions, limitations as to dividends, qualifications, and terms and conditions of redemption. See “Description of Capital Stock” in the accompanying prospectus. We will furnish a full statement of the relative rights and preferences of each class or series of our stock which has been so designated and any restrictions on the ownership or transfer of our stock to any shareholder upon request and without charge.

INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE

The SEC allows us to incorporate into this prospectus supplement information we file with the SEC in other documents. The information incorporated by reference is considered to be part of this prospectus supplement, and information we later file with the SEC will, subject to the next sentence, automatically update and supersede this information. We incorporate by reference the documents listed below and any future filings made with the SEC under Sections 13(a), 13(c), 14 and 15(d) of the Exchange Act until all of the shares of common stock that are part of this offering have been sold or this offering has been terminated. The documents we have incorporated by reference are:

•

Annual Report on Form 10-K for the fiscal year ended December 31, 2025, filed with the SEC on February 26, 2026 (the “2025 Form 10-K”);

•

those portions of our definitive proxy statement on Schedule 14A, filed on April 1, 2025, that are incorporated by reference into our Annual Report on Form 10-K for the fiscal year ended December 31, 2024;

•

Current Report on Form 8-K filed with the SEC (other than information furnished rather than filed) on March 2, 2026; and

•

the description of our common stock contained in our Registration Statement on Form 8-A/A, filed with the SEC on November 15, 2016, and any amendment or report filed for the purpose of updating such description, including the description of capital stock contained in Exhibit 4.5 to our Annual Report on Form 10-K for the fiscal year ended December 31, 2024, incorporated by reference into our 2025 Form 10-K.

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We incorporate by reference any future filings made with the SEC under Sections 13(a), 13(c), 14 or 15(d) of the Exchange Act between the date of this prospectus supplement and the termination of the offering.

Notwithstanding the foregoing, unless specifically stated to the contrary, information that we furnish (and that is not deemed “filed” with the SEC) under Items 2.02 and 7.01 of any Current Report on Form 8-K, including the related exhibits under Item 9.01, is not incorporated by reference into this prospectus supplement or the registration statement of which this prospectus supplement is a part.

Any statement contained in a document that is incorporated by reference will be modified or superseded for all purposes to the extent that a statement contained in this prospectus supplement, the accompanying prospectus or in any other document that is subsequently filed with the SEC and incorporated by reference into this prospectus supplement, modifies or is contrary to that previous statement. Any statement so modified or superseded will not be deemed a part of this prospectus supplement, except as so modified or superseded. Since information that we later file with the SEC will update and supersede previously incorporated information, you should look at all of the SEC filings that we incorporate by reference to determine if any of the statements in this prospectus supplement or in any documents previously incorporated by reference have been modified or superseded.

Upon request, we will provide, without charge, to each person, including any beneficial owner, to whom a copy of this prospectus supplement is delivered, a copy of the documents incorporated by reference into this prospectus supplement but not delivered therewith. You may request a copy of these filings, and any exhibits we have specifically incorporated by reference as an exhibit in this prospectus supplement, at no cost by writing us at the following address: H2O America, 110 West Taylor Street, San Jose, CA 95110 or calling us at (408) 279-7800.

You may also access these documents, free of charge on the SEC’s website at www.sec.gov or on our website at www.h2o-america.com. Information contained on our website is not incorporated by reference into this prospectus supplement and you should not consider any information on, or that can be accessed from, our website as part of this prospectus supplement.

This prospectus supplement is part of a registration statement we filed with the SEC. We have incorporated exhibits into this registration statement. You should read the exhibits carefully for provisions that may be important to you.

You should rely only on the information incorporated by reference or provided in this prospectus supplement. We have not authorized anyone to provide you with different information. We are not making an offer of these securities in any state where the offer is not permitted. You should not assume that the information in this prospectus supplement, the accompanying prospectus or in the documents incorporated by reference is accurate as of any date other than the date on the front of this prospectus supplement or those documents.

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CAUTIONARY STATEMENT CONCERNING FORWARD-LOOKING STATEMENTS

Statements contained or incorporated by reference in this prospectus supplement and the accompanying prospectus that are not statements of historical fact are “forward-looking statements” within the meaning of Section 27A of the Securities Act of 1933, as amended (the “Securities Act”), and Section 21E of the Securities Exchange Act of 1934, as amended (the “Exchange Act”). All statements other than statements of historical fact included in this prospectus supplement and the accompanying prospectus are forward-looking statements made in good faith by us and are intended to qualify for the safe harbor from liability established by the Private Securities Litigation Reform Act of 1995. Forward-looking statements are based on management’s beliefs as well as current expectations, estimates, forecasts, and projections about H2O America and its subsidiaries and the industries in which H2O America and its subsidiaries operate and the beliefs and assumptions of management. Some of these forward-looking statements can be identified by the use of forward-looking words such as “believes,” “expects,” “projects,” “plans,” “estimates,” “anticipates,” “intends,” “seeks,” “plans,” “will,” “may,” “should,” “approximately,” or “strategy,” or the negative of those words or other comparable terminology. These forward-looking statements are only predictions and are subject to risks, uncertainties and assumptions that are difficult to predict. Therefore, actual results may differ materially and adversely from those expressed in any forward-looking statements. The accuracy of such statements is subject to a number of risks, uncertainties and assumptions including, but not limited to, the following factors:

•

the risks associated with the proposed transactions with Quadvest, including the risk of the proposed transactions not closing on the anticipated timeline, or at all, the ability to obtain required regulatory approvals, and the ability to successfully integrate Quadvest’s operations and realize the projected financial and other benefits of the proposed transactions;

•

the effect of water, utility, environmental and other governmental policies and regulations, including regulatory actions concerning rates, authorized return on equity, authorized capital structures, capital expenditures, per- and polyfluoroalkyl substances and other decisions;

•

changes in demand for water and other services;

•

unanticipated weather conditions and changes in seasonality including those affecting water supply and customer usage;

•

the effect of the impacts of climate change;

•

unexpected costs, charges or expenses;

•

our ability to successfully evaluate investments in new business and growth initiatives;

•

contamination of our water supplies and damage or failure of our water equipment and infrastructure;

•

the risk of work stoppages, strikes and other labor-related actions;

•

catastrophic events such as fires, earthquakes, explosions, floods, ice storms, tornadoes, hurricanes, terrorist acts, physical attacks, cyber-attacks, epidemic or similar occurrences;

•

changes in general economic, political, business and financial market conditions;

•

the ability to obtain financing on favorable terms, which can be affected by various factors, including credit ratings, changes in interest rates, compliance with regulatory requirements, compliance with the terms and conditions of our outstanding indebtedness, and general market and economic conditions; and

•

legislative and general market and economic developments.

Additional information concerning these and other risk factors is also contained in our 2025 Form 10-K, which is incorporated by reference herein, as updated by other reports and documents we file with the SEC. We do not undertake any obligation to update or revise the information contained in this prospectus supplement or the accompanying prospectus, any related free writing prospectus or the information included or incorporated by reference herein and therein, including the forward-looking statements, to reflect any event or circumstance that may arise after the date of this prospectus supplement.

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SUMMARY

This summary highlights information contained in or incorporated by reference into this prospectus supplement and the accompanying prospectus. Because this is a summary, it may not contain all of the information that is important to you. Before making an investment decision, you should read this entire prospectus supplement and the accompanying prospectus and the information included or incorporated by reference, including the section entitled “Risk Factors” in this prospectus supplement and such risk factors as may be updated from time to time in our public filings.

H2O America

H2O America is a holding company whose primary business involves ownership of public utilities that provide water and wastewater services, including the production, purchase, storage, purification, distribution, wholesale and retail sale of water. The water utility business of H2O America is conducted through five wholly owned subsidiaries: San Jose Water Company (“SJWC”), Connecticut Water Company (“CWC”), The Maine Water Company (“MWC”) and SJWTX, Inc. doing business as Texas Water Company (“TWC”). These subsidiaries provide water service to approximately 405,000 connections and wastewater service to approximately 4,000 connections and serve a combined population of over 1.6 million people in California, Connecticut, Maine and Texas. Water utility services provided by these businesses are subject to regulation by the applicable state public utility commissions. These subsidiaries also engage in non-tariffed operations that are not subject to public utility commission regulation, including contract water and sewer operations, maintenance agreements, and antenna site leases. Collectively, these water utility and utility-related services form our “Water Utility Services” segment.

Water Utility Services, excluding non-tariffed activities, are subject to rate regulation based on cost recovery. The ratemaking process is intended to provide revenues sufficient to recover normal operating expenses, provide funds for replacement of water infrastructure and produce a fair and reasonable return on stockholder common equity.

•

In California, SJWC’s rates, service and other matters affecting its business are subject to regulation by the California Public Utilities Commission.

•

In Connecticut, CWC’s rates service and other matters affecting its business are subject to regulation by the Public Utilities Regulatory Authority of Connecticut.

•

In Texas, TWC’s rates are subject to the economic regulation of the Public Utility Commission of Texas (the “PUCT”).

•

In Maine, MWC’s rates, service and other matters affecting its business are subject to regulation by the Maine Public Utilities Commission.

See “Business — Regulation and Rates” in our 2025 Form 10-K.

The principal executive offices of H2O America are located at 110 West Taylor Street, San Jose, California 95110. Its telephone number is (408) 279-7800, and its Internet address is www.h2o-america.com. The information on, accessible from or hyperlinked to, our website (other than the SEC filings expressly incorporated herein under “Where You Can Find More Information”) is not incorporated by reference in this prospectus or any accompanying prospectus supplement, and you should not consider it a part of this prospectus or any accompanying prospectus supplement.

Quadvest Acquisition

On July 7, 2025, the Company, through its indirect subsidiary, TWC, entered into an Asset Purchase Agreement (the “Regulated Business APA”), with Quadvest, L.P., a Texas limited partnership, as seller (“Quadvest Retail”), and the Company, as guarantor, pursuant to which, and subject to the terms and conditions set forth therein, Quadvest Retail has agreed to sell, and TWC has agreed to acquire, substantially all of the assets of Quadvest Retail related to the operation of Quadvest Retail’s water and sewer utility business (the “Regulated Business”) at a purchase price consisting of a base amount of $483.6 million, with certain adjustments based on capital expenditures (the “Regulated Business

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Transaction”). The Regulated Business Transaction will be subject to the satisfaction of various closing conditions set forth in the Regulated Business APA, including satisfaction of the closing conditions with respect to the Wholesale Business Transaction described below.

Concurrently on July 7, 2025, the Company, its indirect subsidiary, Texas Water Operation Services, LLC (“TWOS”) and TWC (together with the Company and TWOS, the “Purchasers”), entered into another Asset Purchase Agreement (the “Wholesale Business APA” and, together with the Regulated Business APA, the “Agreements”), with Quadvest Retail and its affiliate, Quadvest Wholesale, LLC, a Texas limited liability company (“Quadvest Wholesale” and, together with Quadvest Retail, the “Sellers”), pursuant to which, and subject to the terms and conditions set forth therein, Quadvest Wholesale has agreed to sell, and TWOS has agreed to acquire, substantially all of the assets of Quadvest Wholesale related to the operation of Quadvest Wholesale’s wholesale water and sewer business (the “Wholesale Business” and together with the Regulated Business, the “Businesses”) at a purchase price consisting of a base amount of $56.4 million, with certain adjustments based on capital expenditures (the “Wholesale Business Transaction” and, together with the Regulated Business Transaction, the “Transactions”). The Wholesale Business Transaction will be subject to the satisfaction of various closing conditions set forth in the Wholesale Business APA, including the simultaneous closing of the Regulated Business Transaction described above.

On July 9, 2025, TWC filed a request with the PUCT to use Fair Market Value (“FMV”) to support its acquisition of Quadvest, L.P. On August 7, 2025, the PUCT appointed three appraisers to determine the FMV. In December 2025, TWC received the appraised FMV from the three PUCT appointed appraisers for the assets of Quadvest, L.P. In accordance with Texas’ FMV statute, the purchase price of $483.6 million will serve as the ratemaking rate base. TWC filed its Sale, Transfer & Merger application with the PUCT in January 2026.

We expect to close the Quadvest Acquisition in mid-2026. However, we may be unable to consummate the Quadvest Acquisition on a timely basis or at all. See “Risk Factors — Risks Relating to Quadvest Acquisition” in our 2025 Form 10-K. This offering is not conditioned on the consummation of the Quadvest Acquisition or the Debt Financing.

Quadvest and the Anticipated Benefits of the Quadvest Acquisition

We believe the Quadvest Acquisition is a strategically compelling transaction that will significantly expand and diversify our footprint in Texas. Quadvest operates mainly in counties surrounding Houston, Texas,many of which have experienced double-digit population growth over the past five years. Quadvest Retail is comprised of 64 subdivisions, which serve 38,000 connections and has a pipeline of 61,500 connections under contract. We believe Quadvest Retail’s excellent relationships with leading developers in the Houston region will continue to drive future growth. Quadvest Wholesale is comprised of 5 subdivisions, which serve 16,500 connections and has a pipeline of 25,500 connections under contract. Quadvest Wholesale constructs, operates and maintains water and wastewater facilities under 30-year or longer contracts with municipal utility districts. We believe the Quadvest Acquisition will bring operational scale, a strong development pipeline and increased exposure to the high-growth Houston market as well as a robust greenfield development strategy with national homebuilders that can be leveraged in other fast growing areas within our service territory. From 2020 through 2025, the compound annual growth rate of Quadvest Retail’s connections has been approximately 10%. From 2022 through 2025, the compound annual growth rate of Quadvest Wholesale’s connections has been approximately 82%.

Quadvest also brings significant local talent and a strong legacy of local relationships and reliable service. It has been providing water and sewer service in Southeast Texas for nearly 50 years through its operating entities.

We expect the Quadvest Acquisition to increase the scale, geographic diversity, rate base and financial foundation of the Company to create an enhanced platform for long-term growth. After giving effect to the Quadvest Acquisition, Texas customers are expected to comprise approximately 18% of H2O America’s total customer connections, based upon customers as of December 31, 2025, with approximately half of our customers in California, approximately one-quarter in Connecticut and the

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remainder in Maine. On the pro forma basis described under “Unaudited Condensed Combined Pro Forma Financial Information” in this prospectus supplement, the combined company had assets of approximately $5.7 billion as of December 31, 2025 and operating revenues of approximately $841.2 million for the year ended December 31, 2025. The combined company is expected to create the second largest regulated private Texas water and wastewater utility. We believe the combined company will be well-positioned for growth in the “infrastructure replacement era” with a footprint in four states that have constructive regulatory environments supporting infrastructure investments.

We expect the Quadvest Transaction to be meaningfully accretive to our earnings per share growth rate over time. Initially, we estimate that the Quadvest Acquisition will have a dilutive effect of 10% to 20% in 2026 and 2027 on our earnings per share based on Adjusted Net Income until the company is able to reflect the ratemaking rate base in rates. See “— Summary Unaudited Condensed Combined Pro Forma Financial Information” in this prospectus supplement and “— Supplemental Unaudited Historical Financial Information — Non-GAAP Financial Measures” for a discussion of Adjusted Net Income, which is a non-GAAP financial measure. Based on the FMV determination described above, the purchase price of $483.6 million will serve as the ratemaking rate base for the Quadvest Retail business, and we expect to submit a rate case application to the PUCT in early 2027 to establish rates based on the ratemaking rate base for that business. Until revised rates are established for the Quadvest Retail business, which we expect to occur in 2028, our earnings per share will not fully reflect the anticipated benefits of the Quadvest Acquisition.

Although we expect that the Quadvest Acquisition will result in benefits to the Company, we may be unable to consummate the Quadvest Acquisition on a timely basis or at all, and if the Quadvest Acquisition is consummated, we may not realize the anticipated benefits because of integration difficulties and other challenges. See “Risk Factors — Risks Relating to Quadvest Acquisition” in our 2025 Form 10-K and “Cautionary Statement Concerning Forward-Looking Statements” in this prospectus supplement.

Financing of the Quadvest Acquisition

In addition to this offering, we expect to finance the remaining portion of the purchase price of the Quadvest Acquisition with net proceeds from up to $140 million of additional debt, which may include the issuance of $100 million of private placement notes of the Company, and other debt financing. We refer to any debt financing that we expect to incur to fund all or any portion of the Quadvest Acquisition, and to pay related fees and expenses, as the “Debt Financing.” If and to the extent that this offering is not completed or is completed for less proceeds than anticipated, we would expect to fund any shortfall with proceeds of the issuance of common stock under our at-the-market offering program or additional Debt Financing.

This offering is not conditioned on the closing of any Debt Financing, and neither this offering nor any Debt Financing is conditioned on the consummation of the Quadvest Acquisition. We may not complete the Quadvest Acquisition or any Debt Financing on the terms contemplated by this prospectus supplement or at all. This prospectus supplement is not an offer to sell or a solicitation of an offer to buy any securities that may be offered in the Debt Financing.

Summary Unaudited Condensed Combined Pro Forma Financial Information

The Unaudited Condensed Combined Pro Forma Balance Sheet gives effect to this offering, the expected Debt Financing and the Quadvest Acquisition as of December 31, 2025 as if they had been completed on December 31, 2025, and the Unaudited Condensed Combined Pro Forma Statement of Operations gives effect to this offering, the expected Debt Financing and the Quadvest Acquisition for the year ended December 31, 2025 as if they had been completed on January 1, 2025, and have been derived primarily from the December 31, 2025 audited consolidated financial statements of H2O America included in H2O America’s Annual Report on Form 10-K for the fiscal year then ended and the audited financial statements of Quadvest as of December 31, 2025 and for the year then ended. Since the Unaudited Condensed Combined Pro Forma Financial Information has been prepared based upon preliminary estimates, the final amounts recorded at the date of the Quadvest Acquisition may

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differ materially from the information presented herein. These preliminary estimates are subject to change pending further review of the assets acquired and the liabilities assumed.

The Quadvest Acquisition is reflected in the Unaudited Condensed Combined Pro Forma Financial Information as an acquisition of Quadvest by H2O America, based on the guidance provided by United States generally accepted accounting principles, or “GAAP,” for business combinations.

Quadvest’s regulated operations are comprised of water and sewer utility activities. The operations are subject to the rate-setting authority of the PUCT and are accounted for pursuant to GAAP, including the GAAP accounting guidance for regulated operations. The rate-setting and cost recovery provisions currently in place for Quadvest’s regulated operations provide revenues derived from costs, including a return on investments of assets included in rate base.

(in thousands, except per share data)	For the Year Ended December 31, 2025
Pro Forma Combined Results of Operations:
Operating revenues		$	841,191
Total operating expenses			690,969
Operating income			150,222
Interest expense, other income and expense			(61,777)
Net income		$	82,628
Pro Forma Combined Per Share Data:
Earnings per share-basic		$	1.83
Earnings per share-diluted		$	1.82

(in thousands)	As of December 31, 2025
Pro Forma Combined Balance Sheet:
Utility plant and intangible assets		$	5,590,551
Less accumulated depreciation and amortization			1,120,232
Net utility plant			4,470,319
Total assets		$	5,704,928
Pro Forma Capitalization:
Total stockholder’s equity		$	2,069,270
Long-term debt (consolidated, excluding current maturities)		$	1,866,819
Total capitalization		$	3,936,089

Supplemental Unaudited Historical Financial Information — Non-GAAP Financial Measures

We present Adjusted Net Income and Funds From Operations (“FFO”), which are non-GAAP financial measure, for the periods set forth below. Our net income is prepared in accordance with GAAP and represent the earnings as reported to the SEC. Adjusted Net Income is a non-GAAP financial measure representing GAAP earnings adjusted to exclude the effects of non-utility real estate transactions and costs associated with mergers and acquisition activities, if any. Adjusted Net Income is provided as additional information for investors to evaluate the performance of our business activities excluding these items. Our management also believes these non-GAAP financial measures help investors and analysts better understand our actual results. We use Adjusted Net Income as one of our primary performance measurements when communicating with analysts and investors regarding our outlook and results. Adjusted Net Income is also used internally to measure performance. Information reconciling Adjusted Net Income to GAAP net income as used in this prospectus supplement is unavailable to us without unreasonable effort.

FFO is calculated as operating income plus depreciation and amortization, losses from non-utility real estate transactions, costs associated with mergers and acquisition activities, impairment of long-lived

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assets, share based compensation expenses, and other income from cash-based recurring unregulated operations; less gain from non-utility real estate transactions, cash taxes paid and cash interest paid. Our management believes this non-GAAP financial measure helps investors and analysts further evaluate our operating performance.

However, these non-GAAP financial measures may be different from non-GAAP financial measures used by other companies, even when the same or similarly titled terms are used to identify such measures, limiting their usefulness for comparative purposes. Further, these non-GAAP financial measures should be considered as a supplement to the financial information prepared on a GAAP basis rather than an alternative to the respective GAAP financial measures.

	For the Year Ended December 31,
(in millions)	2025			2024			2023
Operating income		$	177.5		$	170.5		$	149.4
Depreciation and amortization			115.3			112.9			105.9
Share based compensation expense			6.4			5.6			4.6
Other income/expense			—			—			2.3
(Gain)/loss on sale of real estate investments (a)			(0.3)			0.6			—
Expenses for mergers and acquisition related activities (b)			3.5			3.4			—
Taxes on (a) and (b) above			(0.9)			(1.1)			—
Cash taxes (paid)/received			(3.2)			0.6			(23.0)
Cash interest paid			(79.4)			(73.3)			(67.5)
Total funds from operations (FFO)		$	218.9		$	219.2		$	171.7
Total balance sheet debt		$	1,977.2		$	1,829.7		$	1,747.2
Adjustments:
Cash and liquid investments

SEC Form 424B5 filed by H2O America