According to the link I gave you average retail for that boat is around $40,000. I think if you got it for that out the door it would be a good deal.
It is hard to say how much the price may drop in the off season. It may not go much lower if it is not costing them much to keep it.
Here is a handy loan amortizer:
http://www.yona.com/loan/Plug in your loan amount and choose how many payments and it tells you the payment amount as well as how much interest you will pay.
I just plugged in 40,000 and 15 yrs for a payment of around $350. That gets you paying over 24,000 in interest. The same loan for 7 yrs has a payment of around $550 and only costs 11,000 in interest.
I would look hard at the financing since it could cost alot more than you stand to gain by making a good deal on the selling price.