11 ft Waverunner in 12 ft TH Garage

I'd chock the trailer wheels, strapping it alone might allow more movement the you want.

I would use a 4-wheel dolly with large inflatable wheels and a removable tow bar for your water toy. As others have posted, a winch inside the toy hauler would get it up and down the ramp safely and your truck could still launch and retrieve it off the dolly.

Strapping your Waverunner down to the town hauler while sitting on the dolly instead of trying to anchor the dolly with the toy strapped on it would prevent any movement while in transit.

I would used a 12 volt winch mounted in the garage and pull the trailer in backwards. You can buy a breakaway flange for your trailer tongue to shorten in once trailer is in garage.
Im just wondering if 6" total clearance in length is adequate. Guess if I strap it down good enough it should move at all while underway.

Aquacart 4 play is a common brand for moving jet ski's by hand. one person launch even in soft sand. As long as you surpass the space available you should be able to put it in and secure it. corrugated sheet plastic can be used to slide it in the th, and then slide it out onto the aquacart. ,
Happy motorin.

Can you back into the water and ride it in?

I was thinking of trying to do something similar if I get a hauler. The question I kept asking myself is how would I get it in? Push it in backwards and I doubt the wife and I would be able to push it
Up the incline