First - if you can't afford to pay cash for it - you can't afford the boat. I know that there are hundreds of thousands of people out there who buy on credit but that's pretty well over now folks.
I have found some amenable actually. I checked through a broker for a thirty thousand dollar boat and the broker said the owner would ok with a 50% down payment. Another owner wanted a 20% downpayment. Its a buyers market and they can only say no.
This tells me that the first guy is willing to take 50% less than asking and if you actually do come across with anything over that he's thinking it's gravy. Go back and tell him that you'll give him the 50% cash right now.
But back to my original point. Buy what you can afford. It's going to be a long time before your boat is worth anywhere near what you paid for it.