Investing In America: Unlocking the Harbor Maintenance Trust Fund Act This bill amends the Water Resources Development Act of 1986 and the Internal Revenue Code to make certain amounts in the Harbor Maintenance Trust Fund equivalent to port use tax receipts available for its statutorily prescribed purposes without further appropriation, including for expenditures to pay 100% of the eligible operations and maintenance costs of specified portions of the Saint Lawrence Seaway and up to 100% of the eligible operations and maintenance costs assigned to commercial navigation of all U.S. harbors and inland harbors. Trust fund receipts and disbursements shall: not be counted as new budget authority, outlays, receipts, or deficit or surplus for purposes of the federal budget, the congressional budget, the Balanced Budget and Emergency Deficit Control Act of 1985, or the Statutory Pay-As-You-Go Act of 2010; be exempt from any general budget limitation imposed by statute on government expenditures and net lending (budget outlays); and be available only for the statutorily prescribed purposes of such fund.