Full Utilization of the Harbor Maintenance Trust Fund Act This bill provides a discretionary spending limit adjustment for full utilization of the Harbor Maintenance Trust Fund to ensure that funds are used to support navigation and maintain federally authorized harbors. The bill makes certain amounts in the trust fund available, without appropriation, to pay 100% of the eligible operations and maintenance costs of specified portions of the Saint Lawrence Seaway (a system of locks, canals, and channels in Canada and the United States that permits oceangoing vessels to travel from the Atlantic Ocean to the Great Lakes); and up to 100% of the eligible operations and maintenance costs assigned to commercial navigation of all U.S. harbors and inland harbors. The discretionary spending adjustment in a given fiscal year may not exceed the amount within the Harbor Maintenance Trust Fund on the last day of the fiscal year that is two years prior to such fiscal year.