I would only buy a used boat (which I just did) from a dealer, which I know, with a contract that includes a reasonable warranty period.
Surveys and inspections are helpful but their agreements always disclaim any liablity. The only way to get things covered is in a sales agreement with someone who has the assets (and reputation) to honor that agreement. You need an agreement that puts any "reasonable" responsibility back on the seller (if I were the seller, I'd factor this into my price, which dealers do).
So, its pay me now, or pay me later.
Just my .02
That being said, I doubt I'll ever buy another "new" boat