If wax doesn't stand a chance, how come the green algae and other junk that adheres to the bottom during the season does?
Again, we don't go crazy, but we do put 1 coat of wax on the underside (the parts we can reach). In my view, it seems to retard the junk from growing on the underside, and seems to make it easier to clean off at the end of the season. Your mileage may vary...