To amend the Maritime Act of 1981 to enhance interstate and foreign commerce and improve competitiveness of United States ports in such commerce by establishing a port improvement revolving loan program to be administered jointly by the Secretary of Commerce and the Secretary of Transportation.

Redesignates the Maritime Act of 1981 as the Maritime Act of 1991 and adds a new title: the Port Improvement Act of 1991. Directs the Secretary of Transportation, through the Federal Maritime Administration and the Secretary of Commerce (the Secretaries), to jointly establish a program to make loans to port agencies for financing and refinancing improvements to facilities at deep-draft ports.

Limits the aggregate amount of loans to: (1) a specified dollar amount in a five-fiscal-year period; and (2) 50 percent of the cost of the improvements. Requires the remaining percentage to come from non-Federal sources. Prohibits loans for any activity which is eligible for assistance as a water resource project carried out by the Secretary of the Army through the Chief of Engineers.

Sets forth procedures and criteria for selection of loan recipients. Requires a set-aside of 20 percent of amounts appropriated from the Port Improvement Revolving Loan Fund each year for improvements at small deep-draft ports. Limits loans to a maximum of 20 years. Requires repayment at an interest rate which: (1) will ensure that the amount in the Fund will increase at approximately the rate of inflation; and (2) is less than market rates for such loans.

Establishes in the Treasury the Port Improvement Revolving Loan Fund, to consist of amounts: (1) transferred to the Fund equal to specified percentages of customs duties collected during specified fiscal years; (2) deposited in the Fund as repayment of loans made under this Act; and (3) credited to the Fund from interest on and proceeds from the sale or redemption of investment of amounts in the Fund.

Directs the Secretaries to establish a permanent advisory board to advise the Secretaries on matters related to this Act.