All true, but if a substantial un-disclosed defect is found, say delaminated decks, the negotiation will start again, clause or not. It has too, as the finding lowers the value of the boat... and the seller will largely have to suck that fact up.
I'm in the process of (hopefully) buying a boat and this is exactly what has happened. The sale is being handled by a broker, there were a few known issues that I was made aware of, and after looking at the boat I made my offer, which was accepted. The survey found high moisture readings on the deck, and one area in particular that the surveyor believed was delaminated. All parties agreed to have a boatyard (one that we both had used before) look at the boat and provide an estimate to repair the deck. The yard provided the estimate, they did not believe there was any delamination. Because of the conflicting opinions, the broker arranged for the yard to go ahead and take some core samples to get a clearer picture of the situation (I'm waiting on the results).
The survey turned up a few other issues that prevented doing a sea trial (the boat has been on the hard for a year) and the current owner is going to have those addressed. The estimate that we eventually get from the yard is going to be used to negotiate a new purchase price. In this situation the seller would have to deal with the deck issues, either with this sale now or with someone else who is interested in buying the boat. So saying that the price is firm regardless of what the survey finds is kind of pointless (which the seller I'm dealing with did not do) if he really wants to sell the boat.