You simply back up rapidly toward where the pontoon boat will sit and just before you get there, you slam on your brakes, throw it in drive, and take off the other way. Boat slides off and sits neatly on lawn til spring.
Then in the spring, you invite several of your young strong, heck-ya-we-can-pick-it-up friends over, ply them with wonderful strength giving libations, have them pick it up, and place it on the new trailer. Simple