<PAGE>
 
                                                                    EXHIBIT 10.3

                           Hvide Marine Incorporated
         2200 Eller Drive P.O. Box 13038 Fort Lauderdale, Florida 33316



Donald L. Caldera                             Phone: (303) 523-2200
Executive Vice President Development          Telefax: (303) 527-1772
                                              Direct Line:
                                              524-4200  EXT. 400

July 23, 1996



Board of Directors
Seal Fleet, Inc.
P.O. Box 1168
Galveston, TX 77553

Ladies and Gentlemen:

     We understand that the Board of Directors of Seal Fleet, Inc. (the
"Company") is considering authorizing the Company to waive that portion of the
condition set forth in paragraph 11(b) of the Asset Purchase Agreement between
Hvide Marine Incorporated  ("Hvide Marine"), the Company and certain of its
affiliates, and Thomas M. Ferguson entered into as of March 29, 1996 (the
"Agreement") that requires the exclusion of those shares of Class A Common Stock
owned by Three R Trusts in determining whether the transactions contemplated by
the Agreement have been approved by a majority of the outstanding shares of the
Company's Class A Common Stock (the "Three R Trusts Exclusion").

     In reaching a decision regarding the waiver, we understand that the Board
will consider that the Agreement has been amended, subject to the approval of
the Board, to increase the total consideration to the Company by $3.775 million
by (1) increasing the purchase price of the vessels being sold to Hvide Marine
from $2.1 million per vessel to $3.2 million per vessel, with the additional
proceeds applied to the debt reduction described in the Company's proxy
statement related to the meeting of stockholders scheduled for July 17, 1996
(the "Proxy Statement"), and (2) funding of $475,000 by Hvide Marine to be
utilized by the Company to assist in downsizing its operations, as appropriate,
and in mitigating any dislocation associated with such downsizing.  We
understand further that the Board will also consider, in light of the fact that
the Proxy Statement disclosed that the Company reserved the right to amend the
Agreement before or after approval of the transaction by stockholders and the
fact that the transaction contemplated by the amended Agreement would result in
terms more favorable to the Company and its stockholders than those contemplated
by the transaction described in the Proxy Statement, whether the Company should
resolicit proxies from stockholders by an amended proxy statement prior to
consummation of the transaction contemplated by the amended Agreement.

     In consideration of (i) the Company's adjournment of the 1996 Annual
Meeting of Stockholders ("Meeting") scheduled for July 17, 1996 and (ii) the
Board of Directors' consideration of whether or not to waive the Three R Trusts
Exclusion, Hvide Marine agrees to indemnify, defend, and hold Seal Fleet, Inc.
and each of its Directors and officers who is a party or is threatened to be
made a party to any threatened, pending or completed action, suit, proceeding,
whether criminal, civil, administrative, arbitrative or investigative, any
appeal in such action, suit or proceeding, and any inquiry or investigation that
could lead to such an action suit or proceeding, harmless of and from any and
all claims, actions, demands, losses, costs, 
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Seal Fleet, Inc. Indemnification Letter
July 23, 1996
Page Two

liabilities, penalties, fines, damages and expenses (including, but not limited
to, all court costs and expenses, and reasonable attorneys' fees, whether
incurred in the defense of such claims, suits, actions and demands, or in
enforcing this indemnification provision, whether or not a lawsuit is filed)
(collectively "Costs") arising from or related to (a) the Board of Directors'
consideration, reconsideration and determination whether or not to waive the
Three R Trusts Exclusion, (b) any violation of (i) Section 14 of the Securities
Exchange Act of 1934, (ii) the laws of the State of Nevada, or (iii) any other
applicable federal or state laws, rules or regulations, by Hvide Marine or
Thomas J. Ferguson or any of their respective employees, agents, or
representatives in connection with or in any way related to (x) the solicitation
of proxies for the Meeting and/or (y) the purchase by Hvide Marine, Thomas M.
Ferguson or any of their respective affiliates, agents or representatives of
Company shares; and (c) the Board of Directors' decision not to, and the
Company's not undertaking to, resolicit proxies from the Company's stockholders
by means of an amended proxy statement. Hvide Marine, at its option, will either
assume the defense of any indemnified party at the commencement of any claim,
suit, action or demand with counsel reasonably satisfactory to the indemnified
party or reimburse the indemnified party for all Costs within ten (10) days of
presentation of an invoice for Costs, and any indemnified party will have the
right to invoice the Costs as incurred and need not wait until resolution of the
claim, suit, action or demand.

     This agreement supersedes and replaces in its entirety that certain letter
of indemnity dated July 18, 1996 from Hvide Marine to the Board of Directors of
Seal Fleet, Inc.

                               Sincerely,


                               Hvide Marine Incorporated



                               By:  /s/ Donald L. Caldera
                                   ------------------------------
                                        Donald L. Caldera
                                        Executive Vice President