Whether you buy a new or a used boat, the best way to protect your interests is to outline the terms of the sale in writing. Transfer a boat's ownership with a seller warranty.Use our attorney-drafted boat bill of sale template to outline the terms and conditions clearly. A sale and purchase agreement (SPA) is a legally binding document that details the agreedupon wishes of both the buyer and seller. Learn how to draft a Boat Bill Of Sale in this article, which includes stepbystep instructions. Continue reading to find out more. Every boat transaction I have done has involved a purchase and sale agreement with timelines and performance of buyer and seller outlined. Complete a Bill of Sale which states the purchase price. You'll need this for your municipality to determine tax on the purchase.