Would you elaborate as to what you mean here?
How do you get the seller to assume part of the risk?
Sure, the seller knows the condition of the boat better than anyone else, so I want the seller to make written representations about the condition and his knowledge of the condition before I spend money in reliance on his representations inspecting it.
If it turns out the seller was misrepresenting the condition, then the seller would be liable for the costs of discovering those misrepresentations, i.e., the haul out and marine survey, travel costs, etc.
In fact, if the buyer takes this posture, the dishonest sellers/brokers will refuse to deal with the buyer at the outset, and the buyer saves himself from the costs of a purchase fiasco.
A prime example of this recently was the buyer who bought an expensive boat with extensive keel/hull damage from a grounding - a grounding that was so severe that the seller, or one of his agents, must have known about it.
Some would say "buyer beware"; I say "fraud". When discovered, the seller should reimburse the buyer's damages. The buyer would likely have no recourse against the seller with the standard broker's contract because of the way it is structured and because the boat is sold "as is", so the buyer is out all of his costs due to a dishonest seller/broker.