I've read where some people have used truck bed coating on the bottom. Don't forget the bottom can be coated different than sides and deck.

I don't want to start WWIII but I used tinted epoxy to coat my poly skinned SOF.
Everyone thought this was the exact worse thing to do. That skin is showing some cracks but its over 15 years now. I would run that boat over coral anytime.

Most of the great flexible coatings were very expensive and highly toxic.

You could always use some epoxy just on the bottom. I sanded my skin with some 120 to give it some tooth. Also, to tighten my skin, I used a propane torch and just fanned it over the skin. All skins will loosen up a bit in time. But the nylon skins just get ridiculously loose when wet. I have no idea why nylon is so popular.