Heading for the door is one of the most powerful bargaining tools out there!
But you need to be prepared to actually WALK out that door.
One of the keys to successful negotiations is to not become emotionally involved in your potential
purchase. . . . . . which is easy to plan
, and sometimes not so easy to execute
Edit: One more thought, the Previous Owner of the boat is (or can be) a tremendous asset to the new owner . . . . if you don't burn your bridges during the buying/negotiation process. Easily worth thousands, or sometimes even TENS of thousands of $$$, so that needs to be considered as well . . .