I built a lot of plywood sea kayaks, and the standard is 4mm Okoume with British Standard 1088(BS1088), which is a marine grade. Marine grade is important, not for just the waterproof glue, but also the quality of the wood and lack of voids. I'd recommend the same for plywood surf kayaks or wave skis. 3mm is a lot less strong. You can order it at www.clcboats.com if you can't find a local source.