There is no dealer in the WORLD that would only have a 10% difference between their trade in value and what you can get the boat on the open market. That means you are getting screwed on the new boat. Again...it's all about the difference. The only time it's better to trade-in is when you don't want to mess with selling something out right.
IMO, they will not give you $30k trade for your boat. But, they can roll the remainder of the Supra's loan into the new boat....if there is enough equity. Oh, you gotta love credit....sending people to deep debt every day!