Note that plywood is only painted with gel coat and that it is constantly wetted with bilge water. Close up of view above. It's not hard to see why this piece of plywood would start to rot. I use the same method for all exposed wood and especially plywood. Flat Deck Surfaces.
Most boats have plywood decks located deep in the hull.
These, too, are often just painted with gelcoat. These also prove troublesome because things like pumps, water heaters, hoses and other equipment are screwed down to these decks. Because the decks tend to collect water, the water gets into the screw holes and starts the rot process in the plywood. Obviously, all screw mounting need to be bedded. Also make sure that your water heater pressure relief valve has a drain hose on it, and that it isn't dumping water directly onto the deck on which it is mounted.