Chall....if the boat is already registered previous owners will be available from the registrar. I know this is the case down here and in UK and any Commonwealth or ex Commonwealth countries, I'm presuming it is the same in the USA
I'm a noob, so take with a grain of salt, but I'm fairly sure this is not true. I've heard a couple of times that the US Coast Guard only keeps records for the most recent two names/owners of a vessel. If it has had more than that, you may be SOL. State registration with the US may go back further, and there may be tax records or such... Anyways, this is besides the point because the USA isn't involved in the OP's transaction anyways.