Made in America Shipbuilding Act of 2018 This bill: (1) prohibits an executive agency from entering into a contract for the acquisition, construction, or conversion of a vessel unless the vessel is to be constructed or converted in the United States, and (2) lists components that may be procured for vessels only if manufactured in the United States. An agency may waive such limitation upon determining that: (1) application of the limitation would increase the cost of the overall acquisition by more than 25% or cause unreasonable delays to be incurred, would result in the existence of only one domestic source for the item, or is not in U.S. national security interests; or (2) satisfactory quality items manufactured by a domestic entity are not available or domestic production of such items cannot be initiated without significantly delaying the project. An agency may not (with exceptions) enter into a contract: (1) for the procurement of end items or components for ships that contain a specialty metal not melted or produced in the United States; and (2) for the construction of a vessel unless the steel, iron, aluminum, and manufactured products to be used in the construction are produced in the United States. If a court or federal agency determines that a person intentionally affixed a label bearing a "Made in America" inscription to specified products sold in or shipped to the United States that were not made domestically, that person shall be debarred from contracting with the government for at least five years.