Marine grade plywood just means the fibers are compressed tighter together, there isn't as much empty space in the board for water to collect. Its alot of money extra for (in my opinion) not alot of extra function. Sure, it will hold up better, but it isn't going to be a cure all. I sealed my deck made of regular plywood with Thompson's Water Sealer and its held up great going into its third season. Let me be the first to tell you, it has been SOAKED! But thats another story... By the way, if your hull is aluminum, the chemicals used in wolminized lumber will bleed out when wet and that mix will eat through aluminum.