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                                                                     EXHIBIT 5.1
    
 
   
                          [ALSTON & BIRD LLP LETTERHEAD]
    
 
   
                                  April 14, 1997
    
 
   
  Board of Directors
    
   
  Seacoast Banking Corporation
    
   
    of Florida
    
   
  815 Colorado Avenue
    
   
  Stuart, Florida 34994
    
 
   
  Gentlemen:
    
 
   
     This opinion is given in connection with the filing by Seacoast Banking
Corporation of Florida, a corporation organized and existing under the laws of
the State of Florida ("Seacoast"), with the Securities and Exchange Commission
under the Securities Act of 1933, as amended, of a Registration Statement on
Form S-4 ("Registration Statement") with respect to the shares of the $.10 par
value Class A common stock of Seacoast Common Stock ("Seacoast Common Stock") to
be issued in connection with the proposed merger of Port St. Lucie National Bank
Holding Corp. ("PSHC") with and into Seacoast (the "Merger").
    
 
   
     The Merger is intended to be effected pursuant to an Agreement and Plan of
Merger, dated as of February 19, 1997 (the "Merger Agreement"), by and between
PSHC and Seacoast, pursuant to which each outstanding share of the common stock
of PSHC ("PSHC Common Stock") (excluding shares held by PSHC or any of its
subsidiaries or by Seacoast or any of its subsidiaries, in each case other than
in a fiduciary capacity or as a result of debts previously contracted, and
excluding shares held by shareholders who perfect their dissenters' rights) and
each outstanding PSHC stock warrant will be converted into and exchanged for
solely the right to receive that number of shares of Seacoast Common Stock as
determined in accordance with the terms of the Merger Agreement.
    
 
   
     In rendering this opinion, we have examined the corporate records and
documents, including the Merger Agreement, as we have deemed relevant and
necessary as the basis for the opinion set forth herein. Except to the extent
expressly set forth herein, we have made no independent investigations with
regard thereto, and, accordingly, we do not express any opinion as to matters
that have been disclosed by independent verification. Insofar as the opinions
set forth herein involve the laws of the State of Florida, we have relied on the
opinion of Crary, Buchanan, Bowdish, Bovie, Lord, Roby & Evans Chartered.
    
 
   
     Baed upon the foregoing and subject to the limitations set forth herein, it
is our opinion that the shares of Seacoast Common Stock included in the
Registration Statement have been duly authorized by all requisite actions on the
part of Seacoast and, upon consummation of the Merger, such shares, when issued
to the holders of PSHC Common Stock and PSHC stock warrants in connection with
the Merger as provided in the Merger Agreement, will be validly issued, fully
paid, and non-assessable.
    
 
   
     Members of this firm are licensed to practice law in the State of Georgia
and before the federal courts having jurisdiction in the State of Georgia, and
we express no opinion with regard to any law other than the laws of the State of
Georgia.
    
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     We hereby consent to the use of the opinions as Exhibits 5 and 8 of this
Registration Statement and to the reference made to the firm under the caption
"Opinions" in the Proxy Statement/Prospectus constituting part of the
Registration Statement. In giving consent, we do not thereby admit that we are
within the category of persons whose consent is required under Section 7 of the
Act or the rules and regulations of the Commission thereunder.
    
 
   
                                          Sincerely yours,
    
 
   
                                          ALSTON & BIRD
    
 
   
                                          By:   /s/ RALPH F. MACDONALD III
    
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                                                   Ralph F. MacDonald III