Pass on the boat. There is no such thing as a good deal when it comes to buying someone else's problems. If the other person is willing to give you $4,000 to take the boat off his hands, that is what I would consider a good deal. Otherwise, walk away, in my opinion.
Salt is always an issue, regardless. You'll need new heads too.
Too many what-ifs... The guy that said the boat has been well cared for, etc??? Poo-Poo....and a lot of it..