The purpose of the Sustainable Vessel Fuel Act is to encourage the production and use of sustainable fuel for commercial vessels and ferries. It aims to modify existing tax credits related to clean fuel production, specifically to include a special rate for sustainable vessel fuel. This is part of a broader effort to promote environmentally friendly fuel options and reduce emissions from maritime transportation. Key provisions of the Act include changes to the Internal Revenue Code that expand the definition of eligible clean fuels to include sustainable vessel fuel.