I can't imagine a seller doing 1/2 now 1/2 later and transferring ownership with only 1/2 in hand. Now if you put a 50% deposit down with balance pending survey and told the seller that it would be in 60 to 90 days you might have a chance. Regarding the offer of 10% less is too lean, 25 to 30% under ask is where I would place my initial offer if the boat is up to standards.
Most important issue is there are no rules and it can't hurt to ask.
2007 Hunter 49