<Page> Filed by Seacoast Financial Services Corporation Pursuant to Rule 425 under the Securities Act of 1933, as amended. Subject Company: Bay State Bancorp, Inc. Commission File No.: 001-13691 [SEACOAST LOGO] ACQUISITION OF [BAY STATE BANCORP, INC. LOGO] ================================================================================ December 19, 2002 <Page> FORWARD LOOKING STATEMENT This presentation contains forward-looking statements within the meaning of Section 27A of the Securities Act of 1933, as amended, and Section 21E of the Securities Exchange Act of 1934, as amended, with respect to the financial condition, results of operations and business of Seacoast Financial Services Corporation following the consummation of the merger that are subject to various factors which could cause actual results to differ materially from such projections or estimates. Such factors include, but are not limited to, the following: (1) the businesses of Seacoast Financial Services Corporation and Bay State Bancorp, Inc. may not be combined successfully, or such combination may take longer to accomplish than expected; (2) expected cost savings from the merger cannot be fully realized or realized within the expected timeframes; (3) operating costs, customer loss and business disruption following the merger, including adverse effects on relationships with employees, may be greater than expected; (4) governmental approvals of the merger may not be obtained, or adverse regulatory conditions may be imposed in connection with government approvals of the merger; (5) the stockholders of Bay State may fail to approve the merger; (6) adverse governmental or regulatory policies may be enacted; (7) the interest rate environment may further compress margins and adversely affect net interest income; (8) the risks associated with continued diversification of assets and adverse changes to credit quality; (9) competitive pressures from other financial service companies in Seacoast Financial's and Bay State's markets may increase significantly; and (10) the risk of an economic slowdown that would adversely affect credit quality and loan originations. Other factors that may cause actual results to differ from forward-looking statements are described in Seacoast Financial's filings with the Securities and Exchange Commission. Neither Seacoast Financial nor Bay State undertakes or intends to update any forward-looking statements. [SEACOAST LOGO] 1 [BAY STATE BANCORP, INC. LOGO] <Page> TRANSACTION SUMMARY <Table> Transaction Value: $26.42(1) Aggregate Value: Approximately $137.8 million (including options) Consideration/Election Procedure: 55% Stock / 45% Cash; subject to proration Pricing Mechanism: Exchange ratio of 1.257 on stock consideration and cash consideration of $27 per share are both fixed Collars: None Walk Away: Double trigger - (1) 10% absolute decline as compared to the average closing price of Seacoast's common stock at announcement; and (2) by more than 20% decline relative to an index comprised of peer companies. Estimated Cost Savings: 20% of Bay State's expenses in the first year and 35% in the second year Termination Fee: $6.0 million payable upon termination, 4.3% of deal value of proposed transaction Due Diligence: Completed Board Representation: 2 Bay State Directors to be added to pro-forma total of 16 Seacoast board seats Required Approvals: Bay State shareholders and Customary regulatory approvals Expected Closing: Second quarter of 2003 Pro-Forma Ownership 87% SCFS / 13% BYS </Table> - ---------------- (1) BASED ON SCFS CLOSING PRICE OF $20.64 ON DECEMBER 19, 2002 [SEACOAST LOGO] 2 [BAY STATE BANCORP, INC. LOGO] <Page> SNAPSHOT OF BAY STATE BANCORP BAY STATE'S OPERATIONS AND BUSINESS ARE AN EXCELLENT FIT WITH THOSE OF SEACOAST'S STRONG FINANCIAL PERFORMANCE(1) o 1.40% Return on Average Assets o 13.68% Return on Average Equity o 4.71% Net Interest Margin ATTRACTIVE COMMUNITY BANKING FRANCHISE o $563 million in assets o $484 million in loans o $356 million in deposits (67% transaction and savings accounts) o Approximately 47% of Bay State's loans are multi-family and commercial real-estate o Operates 6 branches in Brookline, Norwood, Westwood, Dedham, Boston and Walpole o Pristine asset quality - ---------------- (1) FISCAL YEAR-TO-DATE FOR THE 6 MONTHS ENDED SEPTEMBER 30, 2002 [SEACOAST LOGO] 3 [BAY STATE BANCORP, INC. LOGO] <Page> SNAPSHOT OF BAY STATE BANCORP FRANCHISE ENHANCING o Consistent with our long-term strategic plan o Extends Seacoast's franchise into "deposit and loan/customer-rich" markets o Provides meaningful entry platform to Boston metropolitan area o Enhances diversification of Seacoast's loan portfolio and deposit base o Further improves our asset quality profile SOLID BALANCE SHEET / GROWTH OPPORTUNITIES o Bay State has excellent asset quality and an attractive deposit mix o Growth potential from: o Higher legal lending limit o Leveraging our expertise in consumer commercial and residential lending o Cross-sell of Seacoast's broader product array and alternative delivery systems [SEACOAST LOGO] [BAY STATE BANCORP, INC. LOGO] <Page> FINANCIALLY ATTRACTIVE (WITH CONSERVATIVE ASSUMPTIONS) o As structured, the transaction is expected to be 1.7%, or 3 cents, accretive to GAAP EPS and 3%, or 5 cents, accretive to cash EPS in the first full year and 8 cents accretive to GAAP EPS and 11 cents accretive to cash EPS in the second year. o Annual Cost savings assumed are $4.4 million (or 35% of Bay State's non-interest expenses with 57% of these achieved in year one and 100% achieved in year two). o No revenue enhancements have been assumed, but many opportunities have been identified. o Mix of cash and stock is a favorable financing option. o Favorably priced relative to comparable transactions. o Effective use of capital. [SEACOAST LOGO] 5 [BAY STATE BANCORP, INC. LOGO] <Page> TRANSACTION RATIONALE LOW LEVEL OF EXECUTION RISK o Seacoast has demonstrated success in executing and integrating similar transactions o Home Port - $328 million total assets o Sandwich - $519 million total assets o No unusual or unfamiliar lines of business o Straight forward balance sheet o Minimal business disruption expected given no branch overlap [SEACOAST LOGO] 6 [BAY STATE BANCORP, INC. LOGO] <Page> OPPORTUNITY FOR EXPANSION - BOSTON METRO AREA [MAP] [SEACOAST LOGO] 7 [BAY STATE BANCORP, INC. LOGO] <Page> DEAL PRICING <Table> <Caption> Pricing of Comparable Transactions(3) ---------------------- Median Mean ---------- ----------- Price / LTM Reported EPS:(1) 18.6x 18.8x 18.6x Price / Estimated Fiscal Year 2004 EPS:(1) 15.5x 17.7x 17.3x Price / Book Value:(2) 212.0% 255.2% 230.7% Price / Tangible Book Value:(2) 212.0% 255.2% 243.2% Based on Seacoast closing price of $20.64 on 12/19/02 with 55% stock and 45% cash mix. </Table> - ---------------- (1) LTM EPS AS REPORTED BY SNL SECURITIES. FY04E EPS OF $1.70 PER SHARE IS I/B/E/S MEDIAN EARNINGS ESTIMATE, BASED ON MARCH 2004 FISCAL YEAR END. (2) BASED ON STATED BOOK VALUE AND INTANGIBLE ASSETS AS OF SEPTEMBER 30, 2002. (3) TRANSACTIONS ANN. SINCE 11/1/01 WHERE SELLER IS A BANK OR THRIFT LOCATED IN MA, CT OR NH. [SEACOAST LOGO] 8 [BAY STATE BANCORP, INC. LOGO] <Page> PRO FORMA IMPACT BASED ON EARNINGS ESTIMATES <Table> <Caption> 2003 -------------- ($ in millions except per share amounts) SCFS Net Income(1) $ 38.5 BYS Net Income(2) 8.3 ---------- Combined Earnings $ 46.8 Cost Savings (after-tax) 1.6 Cost of Cash (1.9) New CDI Intangible Amortization (0.7) Other (0.2) Cost of Asset/Liability Management (0.5) ---------- Pro Forma Earnings* $ 45.2 ========== Pro Forma Cash Earnings(3) $ 46.7 Pro Forma GAAP Earnings $ 45.2 Average Diluted Shares Outstanding 26.8 Pro Forma GAAP EPS $ 1.69 Cash EPS Accretion/(Dilution) 3.0% GAAP EPS Accretion/(Dilution) 1.7% </Table> - ---------------- (1) RBCO estimate of $1.66 (2) I/B/E/S estimate of $1.70 for the fiscal year end March 30, 2004 (3) Pro forma cash earnings adds back approximately $1.5 of amortization of intangibles, which includes existing intangibles and new intangibles from the transaction * May not add due to rounding [SEACOAST LOGO] 9 [BAY STATE BANCORP, INC. LOGO] <Page> ESTIMATED COST SAVES <Table> <Caption> Bay State 20% 35% LTM Year One Year One ------------ ------------- -------------- ($ in millions) SOURCES OF COST SAVINGS Salaries and Benefits $ 8.1 $2.0 $2.8 Other Operating $ 4.5 $0.5 $1.6 ------------ ------------- -------------- Total (Pre-Tax) $12.6 $2.5 $4.4 </Table> [SEACOAST LOGO] 10 [BAY STATE BANCORP, INC. LOGO] <Page> ESTIMATED DEAL COSTS <Table> <Caption> Cost --------------- ($ in millions) Employee-Related $16.5 Conversion / Integration $ 0.8 Transaction Costs $ 2.6 Total Merger Costs (Pre-Tax) $19.9 </Table> [SEACOAST LOGO] 11 [BAY STATE BANCORP, INC. LOGO] <Page> STRONG PRO FORMA FINANCIAL POSITION <Table> <Caption> AT SEPT. 30, 2002 --------------------------- PRO FORMA (000'S) SCFS BYS AT CLOSE (A) ------------- ------------ ------------------ BALANCE SHEET ITEMS: ASSETS $3,722,708 $563,478 $4,324,843 LOANS 2,882,918 490,236 3,373,154 INTANGIBLE ASSETS 35,655 0 134,210 DEPOSITS 2,433,545 356,288 2,789,833 EQUITY 321,604 55,464 412,925 LOAN LOSS RESERVE 33,141 4,748 37,889 CAPITAL SECURITIES 55,017 0 55,017 CAPITAL RATIOS: EQUITY/ASSETS 8.64% 9.84% 9.55% TANG. EQUITY/ASSETS 7.76% 9.84% 6.65% TIER 1 CAPITAL RATIO 13.40% 10.59% 10.85% ASSET QUALITY RATIOS NPLS/LOANS 0.39% 0.03% 0.33% LOAN LOSS RESERVE/NPL'S 297.58% 3252.05% 335.81% LOAN LOSS RESERVE/LOANS 1.15% 0.97% 1.12% </Table> - ---------------- (A) Balance sheet data as of September 30, 2002 as adjusted to include earnings to close, the impact of one-time charges and other purchase accounting adjustments including the stock consideration issued to Bay State. [SEACOAST LOGO] 12 [BAY STATE BANCORP, INC. LOGO] <Page> SOLIDIFIES POSITION AS THE LARGEST PUBLICLY-TRADED MASSACHUSETTS-BASED THRIFT <Table> <Caption> ASSETS 9/30 COMPANY NAME TICKER ($000) --------------------------------------------- ------ --------- SEACOAST PRO FORMA SCFS 4,324,843 ---------------------------------------------------------------- 1 SEACOAST FINANCIAL SERVICES CORPORATION SCFS 3,722,708 2 FIRSTFED AMERICA BANCORP, INC. FAB 2,523,019 3 First Essex Bancorp, Inc. FESX 1,765,905 4 BostonFed Bancorp, Inc. BFD 1,552,493 5 Brookline Bancorp, Inc. BRKL 1,451,318 6 Port Financial Corp. PORT 1,443,634 7 Berkshire Hills Bancorp, Inc. BHL 1,066,642 8 MASSBANK Corp. MASB 1,003,820 9 Abington Bancorp, Inc. ABBK 937,838 10 Westfield Financial Inc. (MHC) WFD 795,397 11 Woronoco Bancorp Inc. WRO 717,659 12 BAY STATE BANCORP, INC. BYS 563,478 </Table> [SEACOAST LOGO] 13 [BAY STATE BANCORP, INC. LOGO] <Page> AN ATTRACTIVE AND COMPLEMENTARY LOAN PORTFOLIO <Table> <Caption> % OF % OF % OF SCFS TOTAL BYS TOTAL PRO FORMA TOTAL ---------- ----- -------- ----- ---------- ----- ($000'S) COMMERCIAL AND INDUSTRIAL $ 121,029 4.2% $ 12,807 2.6% $ 133,836 4.0% REAL ESTATE: CONS. & LAND DEVELOPMENT 114,175 4.0% 29,868 6.0% 144,043 4.3% 1-4 FAMILY RESIDENTIAL 1,419,554 49.4% 204,145 40.9% 1,623,699 48.1% COMMERCIAL MORTGAGES 339,247 11.8% 132,590 26.6% 471,837 14.0% MULTI-FAMILY 14,155 0.5% 103,346 20.7% 117,501 3.5% OTHER 65,353 2.3% 0 0.0% 65,353 1.9% CONSUMER 801,909 27.9% 15,818 3.2% 817,727 24.2% ---------- -------- ---------- TOTAL LOANS $2,875,422 $498,574 $3,373,996 ========== ======== ========== </Table> - ----------------- NOTE: DATA AS OF 9/30/02 [SEACOAST LOGO] 14 [BAY STATE BANCORP, INC. LOGO] <Page> MAINTAINS EXCELLENT ASSET QUALITY <Table> <Caption> SCFS BYS PRO FORMA ---------------- ---------------- ---------------- Non-Performing Assets / Assets 0.33% 0.03% 0.28% Non-Performing Assets / Gross Loans 0.42% 0.03% 0.36% LLR / Non-Performing Loans 297.58% 3252.05% 335.81% LLR / Gross Loans 1.15% 0.97% 1.12% </Table> - --------------- NOTE: DATA AS OF 9/30/02 [SEACOAST LOGO] 15 [BAY STATE BANCORP, INC. LOGO] <Page> DEPOSIT MIX <Table> <Caption> % OF % OF % OF SCFS TOTAL BYS TOTAL PRO FORMA TOTAL ---------- ----- -------- ----- ---------- ----- ($000'S) NON-INT. BEARING DEPOSITS $ 234,002 9.6% $ 23,620 6.6% $ 257,622 9.2% MONEY MARKET & NOW 861,823 35.4% 176,818 49.6% 1,038,641 37.2% PASSBOOK & SAVINGS 311,769 12.8% 37,820 10.6% 349,589 12.5% TERM DEPOSITS 1,025,951 42.2% 118,030 33.1% 1,143,981 41.0% ---------- -------- ---------- TOTAL DEPOSITS $2,433,545 $356,288 $2,789,833 ========== ======== ========== </Table> - --------------- NOTE: DATA AS OF 9/30/02 [SEACOAST LOGO] 16 [BAY STATE BANCORP, INC. LOGO] <Page> SUMMARY o Bay State is a solid, profitable franchise o Logical, low risk entry opportunity to expand into demographically attractive Boston Metro Area markets o Immediately accretive to both new GAAP and cash EPS o Effective use of excess capital versus buyback or de novo branching o Consistent with our strategy of pursuing in-market or contiguous acquisitions of high quality franchises o Bay State has excellent asset quality due to a conservative credit focus o Seacoast has a proven track record of integrating acquisitions [SEACOAST LOGO] 17 [BAY STATE BANCORP, INC. LOGO] <Page> APPENDIX - ADVISORS <Table> <Caption> SEACOAST BAY STATE --------------- ----------------------------- Investment Banking Ryan Beck & Co. Sandler O'Neill & Partners LP Legal Foley Hoag LLP Muldoon Murphy & Faucette LLP </Table> [SEACOAST LOGO] 18 [BAY STATE BANCORP, INC. LOGO]