did you insure it when you bought it?, and did it come with a boat safety certificate (which i think you need to claim any insurance)?
apart from that , i dont know, i did the same at the begining of the year but fortunatly it hasnt sunk yet, but the alternator died, and the engine will not start.
As long as the seller described it accurately, that is all they have to do. It is down to the buyer in a private sale to make adequate enquiries to make sure it is what they want and that it is OK.
If there was any element of false description, then you could try and claim the money back.