Jimgo, excellent advice!!! The way to buy a boat. or, anything else for that matter! Be upfront about the money. Establish that you aren't a lowballer just out to squeeze the seller. And, remind the seller of any benefits you bring to the table. For instance- cash in hand!!!!
I think it depends a lot on which end of the market you are talking about.
Boats for $15K, $150K or >$500K...
The guy with an older 30 footer which market value is $15K might be willing to let it go for $10K , "just to get rid of it" considering slip fees, insurance and maintenance may amount to equal or more than $5K/yr. Also $5K may not feel like such a "hard hit" to the seller.
The guy with a $150K (market value) boat will probably be less likely to sell it for $100K (unless very dire circumstances) since his slip fees, insurance and maintenance will not amount to $50K/yr. Also, for most people, $50K is a significant financial loss.
The guy with the >$500K boat is not going to lose 30-50% on a boat deal. He is smart. and rich. he can just wait.
Broad generalizations, I know ...