I think that the argument about some non-marine plywood is that the middle layer is made from "trash wood" that is there just to take up space, dosen't have the grain oriented properly, and is usually thicker than the face plys, and may have "voids", creating weak spots. I hated to pay $140 for three sheets of 4mm okume marine ply, but I intend to use the kayak on the ocean, and anticipate some rough landings on rocky beaches. I really wonder about the stuff, as it looks like the packaging on the 5lb clementine boxes in the supermarket. Perhaps a compromise would be to use the marine stuff on the hull and 5.5mm (1/4") luan on the deck? Unless you have wholesale sources, you are probably going to pay $300 more for the fiberglass, epoxy, etc. anyway, so you may not want to jeopardize your investment in time and money ... I just didn't have the guts to try something different. Good Luck!