Boats, docks and structures near water commonly use plywood. However, moisture is a big problem for it. If not properly sealed, water can get into the layers of wood which leads to peeling apart, rotting and losing its structure. Using a marine coat on plywood makes a strong shield that prevents water from getting in, protects against damage by UV rays and considerably increases how long the wood lasts. If you miss this step, it could result in costly repairs in a few seasons.