I remember some time back someone took a plastic lawn chair and cut the legs off it and was going to use it in a kayak..didn't look too bad, he painted it to match kayak.
Have you tried L.L. Johnson Lumber in Charlotte,Mi? Their website is www.theworkbench.com. Their website lists several kinds of marine plywood rated bs1088..and there is quite a bit of information about the differences in marine plywood, and they seem to deal quite a bit with boatbuilders. Check it out.