The boat bill of sale is necessary for several reasons. First, it shows the legality of the transaction of the boat and transfers ownership of the vessel and any liens or fines currently accrued to the new owner. The bill of sale is necessary for you as the buyer so that you can register the vessel to you in your state, but you will also need it if you intend to seek documentation from the U.S. Coast Guard, so your vessel becomes part of the federal registry.