The U.S. mil spec 1/8 plywood (I couldn't find marine grade 1/8) is 3 ply, and on both of my sheets that I bought there was a tendency for the plies to come unglued. Might have just been a bad batch. On the other hand, the European (Finnish & German) plywood is 5 or 6 ply (the 1/8th). It seemed much stronger and stiffer. But the Europian plywood doesn't come in 8 foot lengths, the largest size being 60"x60". So if you use it for the sides you have to do more scarf joints. For the scarf joints, I found an air drill with a round sanding disc would take the material off FAST. You just keep the "ply lines" straight and it works well. Good luck. Mark