<PAGE>
 
EXH 10.4

                              TRANSMONTAIGNE INC.
                     TRANSMONTAIGNE PRODUCT SERVICES INC.
                   TRANSMONTAIGNE PRODUCT SERVICES EAST INC.
                 TRANSMONTAIGNE PRODUCT SERVICES MIDWEST INC.
                  TRANSMONTAIGNE TRANSPORTATION SERVICES INC.
                         TRANSMONTAIGNE PIPELINE INC.
                        TRANSMONTAIGNE TERMINALING INC.
                             BEAR PAW ENERGY INC.
                              2750 Republic Plaza
                            370 Seventeenth Street
                            Denver, Colorado  80202



                              AMENDMENT NO. 1 OF
                 SECOND AMENDED AND RESTATED CREDIT AGREEMENT


                                                As of December 14, 1998

BANKBOSTON, N.A.,
 for Itself and as Agent
100 Federal Street
Boston, Massachusetts 02110

CIBC INC.
1600 Smith St., Suite 3100
Houston, Texas  77002

U.S. BANK NATIONAL ASSOCIATION
950 Seventeenth Street, Suite 300
Denver, Colorado  80202

NATIONSBANK, N.A.
Energy Finance Division - Denver
370 Seventeenth St., Suite 3250
Denver, CO  80202

Ladies and Gentlemen:

     Each of TransMontaigne Inc. (the "Company"), TransMontaigne Product
Services Inc. and TransMontaigne Product Services East Inc., each a Delaware
corporation, TransMontaigne Product Services Midwest Inc., TransMontaigne
Transportation Services Inc., TransMontaigne Pipeline Inc. and TransMontaigne
Terminaling Inc., each an Arkansas corporation, and Bear Paw Energy Inc., a
Colorado corporation, hereby agrees with you as follows:
<PAGE>
 
1.   Reference to Credit Agreement and Definitions. Reference is made to the
     ---------------------------------------------    
Second Amended and Restated Credit Agreement dated as of October 30, 1998, as
from time to time in effect, among the Company, certain Guarantors named
therein, BankBoston, N.A., for itself and as Agent, and certain other Lenders
from time to time party thereto (the "Credit Agreement"). Terms defined in the
Credit Agreement and not otherwise defined herein are used herein with the
meanings so defined.

2.   Recital. The parties to the Credit Agreement have decided to modify the
     -------                                                                 
definitions of "Affiliate" and "Eligible Assignee" as used therein.


3.   Amendment. The Credit Agreement is hereby amended, effective as of the date
     ---------
hereof, as follows:


     3.1. Section 1.8 is hereby amended to read in its entirety as follows:

          "Affiliate" means, with respect to the Company (or any other specified
           ---------                                                            
     Person), any other Person directly or indirectly controlling, controlled by
     or under direct or indirect common control with the Company (or with such
     other specified Person), and shall include (a) any executive officer or
     director or general partner of the Company (or of such other specified
     Person) and (b) any Person of which the Company (or such other specified
     Person) or any Affiliate (as defined in clause (a) above) of the Company
     (or of such other specified Person) shall, directly or indirectly,
     beneficially own either (i) at least 25% of the outstanding equity
     securities having the general power to vote or (ii) at least 25% of all
     equity interests; provided, however, that Lion Oil Company, an Arkansas
                       --------  -------                                    
     corporation, shall not be deemed to be an Affiliate of the Company or of
     any Subsidiary of the Company under clause (b) of this definition, unless
     the Company or such Subsidiary shall, directly or indirectly, beneficially
     own either (x) at least 30% of the outstanding equity securities having the
     general power to vote of Lion Oil Company or (y) at least 30% of all equity
     interests in Lion Oil Company.

     3.2. Section 1.50 is hereby amended to read in its entirety as follows:

          "Eligible Assignee" means any of (a) a commercial bank organized under
           -----------------                                                    
     the laws of the United States, or any State thereof or the District of
     Columbia; (b) a savings and loan association or savings bank organized
     under the laws of the United States, or any State thereof or the District
     of Columbia; (c) a commercial bank organized under the laws of any other
     country which is a member of the Organization for Economic Cooperation and
     Development (the "OECD"), or a political subdivision of any such country,
     provided that such bank is acting through a branch or agency located in the
     --------                                                                   
     country in which it is organized or another country which is also a member
     of the OECD; (d) the central bank of 
<PAGE>
 
     any country which is a member of the OECD; (e) an insurance company that is
     engaged in making, purchasing or otherwise investing in commercial loans in
     the ordinary course of its business; (f) an Affiliate of any entity
     described in clause (a), (b), (c), (d) or (e); and (g) any Person which the
     Agent and the Company agree is an Eligible Assignee; provided, however,
                                                          --------  -------  
     that no entity described in clause (a), (b), (c), (d), (e) or (f) above
     shall be an Eligible Assignee unless it has total assets in excess of $1
     billion and unless debt obligations issued by such entity (or by a parent
     entity owning beneficially all of the capital stock of such financial
     institution) are rated "A3" or higher by Moody's or "A-" or higher by S&P.

4.   Representations and Warranties.  In order to induce you to enter into this
     ------------------------------                                            
Amendment, each of the Obligors hereby represents and warrants that each of the
representations and warranties contained in Section 7 of the Credit Agreement is
true and correct on the date hereof.

5.   Conditions to Effectiveness of Amendment.  Acceptance of the foregoing
     ----------------------------------------                              
amendment by the Required Lenders shall be subject, without limitation, to the
condition that no Default or Event of Default under the Credit Agreement shall
have occurred and be continuing.

6.   Miscellaneous.  This Amendment may be executed in any number of
     -------------                                                  
counterparts, which together shall constitute one instrument, shall be a Credit
Document, shall be governed by and construed in accordance with the laws of The
Commonwealth of Massachusetts (without giving effect to the conflict of laws
rules of any jurisdiction) and shall bind and inure to the benefit of the
parties hereto and their respective successors and assigns, including as such
successors and assigns all holders of any Credit Obligation.
<PAGE>
 
     If the foregoing corresponds with your understanding of our agreement,
please sign this letter and the accompanying copies thereof in the appropriate
space below and return the same to the undersigned.  This letter shall become a
binding agreement among each of the Lenders and the Agent when both the Company
and the Agent shall have one or more copies hereof executed by each of the Agent
and the Required Lenders.



                            Very truly yours,

                            TRANSMONTAIGNE INC.

                            By    /s/ Richard E. Gathright             
                               ---------------------------------------------
                               Richard E. Gathright, President

                            TRANSMONTAIGNE PRODUCT SERVICES INC.
                            TRANSMONTAIGNE PRODUCT SERVICES
                               MIDWEST INC.
                            TRANSMONTAIGNE PRODUCT SERVICES EAST INC.
                            TRANSMONTAIGNE TRANSPORTATION
                               SERVICES INC.
                            TRANSMONTAIGNE PIPELINE INC.
                            TRANSMONTAIGNE TERMINALING INC.
                            BEAR PAW ENERGY INC.

                            By    /s/ Richard E. Gathright
                              ----------------------------------------------
                               Richard E. Gathright, Chief Executive Officer
                               of each of the foregoing corporations

The foregoing Amendment is hereby agreed to:

BANKBOSTON, N.A.,
 for Itself and as Agent

By:  /s/ Christopher C. Holmgren        
   -----------------------------        
  Authorized Officer

CIBC INC.

By  /s/ Michael A.G. Corkum            
  ------------------------------       
  Authorized Officer
<PAGE>
 
U.S. BANK NATIONAL ASSOCIATION

By   /s/ Monte E. Deckerd                    
  -----------------------------             
  Authorized Officer

NATIONSBANK, N.A.

By  /s/ David C. Rubenking
  -----------------------------             
  Authorized Officer