You could do both ( resin & mat ) but all you're trying to accomplish is to make it as waterproof as possible. Just a couple of good coats of a polyester resin should be fine. Don't forget the edges !!!
if you are speaking of the back side of the plywood floor. most don't do anything, they screw it down, and just seal the top by glassing over the whole structure. if you do it right, around the edges, and joints, water is not suppose to get there.
