If the seller knew--or had reason to know (that  is, any reasonable seller in his position would have known) of the back taxes and the problem with title--then he committed fraud by misrepresenting (lying about) a material (or significant) fact: his ability to provide clean title. Fraud can provide grounds to sue for monetary compensation (i.e. the amount of the taxes) or to rescind the sale (that is, return boat, get money back).