If you're going to patch or seal something on a boat it's a good idea to use the right material for a long lasting repair. Polyester resin and fiberglass is what your boat is made of and what you should use for hull repairs.
I wouldn't, I suspect a base coat of a soft material like that would lead to the top coat scratching very easily, if you could even get the top coat to bond to it.
The ONLY way I could see that junk being useful on a boat is if you cut a hole in the bottom and mounted a screen door there... THEN yeah use flex seal to coat the screen and go fishing..... but stay close to shore