There's nothing like a new boat
A lot depends on the size style and brand of boat. For a mainstream production boat, I'd use 20-25% off MSRP as a starting point. If you can get around there you are paying what most people pay. A popular brand, a new model, a performance boat and luxury brand may give less discount or make you work harder to get it. An entry-level loss leader might be a lot less. A left-over or stale model should be better.
Any better than that and you have to work at it. If you want to get best prices you can't be in a hurry and have to be prepared to walk away. Car dealer have essentially an infinite supply of cars, they can make money just by moving a lot of cars. Boats take time to order and the season is short. A boat dealer has to maximize profit on each boat he sells. He may be willing to let you get away, even from a profitable deal, if there's a more profitable deal possible.