The main purpose of this legislation, known as the Maritime Fuel Tax Parity Act, is to update tax regulations related to alternative motorboat fuels. Specifically, it aims to extend an existing tax exemption to certain vessels that operate along only one coast of the United States. This change is intended to support the use of alternative fuels in maritime activities and promote environmental sustainability. Key provisions of the legislation include an amendment to the Internal Revenue Code that expands the exemption from excise tax on alternative motorboat fuels.