Just finished up an aluminum bass boat rig yesterday. Had to put 4 batteries, 12 gal tank, on board charger and a ton of wiring back under the rear compartment and there's no easy way to get into the front of it to gain access either on or off the trailer. At first I just climbed in there, feet sticking out of the back of the boat and did what I could. Then as I got out of it for the umpteenth time, I realized that I could use the creeper to get in there better. It worked excellent. Topside creeper that is.
https://www.northerntool.com/shop/tools/product_200360239_200360239
That link is similar to the one I have.
I do have a jet ski dolly outside that would work pretty good for moving a boat around in the shop though. Maybe even in the yard. It's not very long so you'd have to balance it, but it'd work great to slide the boat off the trailer and then work on the trailer, or boat.