A key question is if you signed a Bill of Sale whereby you accepted the item "as is". If so, the doctrine of "Caveat Emptor" (Let the Buyer Beware) may apply and the Bill of Sale arguably will supersede any prior misstatements of fact in the advertising. This doctrine states that a buyer can not recover damages from a seller for defects that make the property unfit for ordinary purposes, except if the seller overtly concealed latent defects or made material misrepresentations tantamount to fraud. In your case, if there is no written Bill of Sale or any other written acknowledgment that you accepted the boat in an "as is" condition, you may be able to recover under: i) breach of express warranty in the advertising, or ii) breach of implied warranty of merchantability whereby goods must be "fit for the intended purpose. You may also seek to rescind the sale agreement if the seller's intentional misstatements about the boat in the offer process constitute fraud.