The boat I built required 4mm Okume but that wood is too expensive. A great alternative is exterior Luan, meaning the glue is waterproof or at least water resistant and therefore a stronger glue. For the deck and sides I used 1/8' Luan that's been coated with epoxy and taped at the seams. Now here's the trick. The bottom is made of two sections of 1/8" Luan that have been glued together. That gives you 6 ply strength (actually 7 ply!!) with only 1/4" thickness. I'm using the same technique on a double kayak I'm just starting on. The 6-ply design makes for an extremely strong boat that is still pretty light, and much cheaper to build. What's more, you can bend two sheet of 1/8" Luan before glueing and achieve almost any shape. And for the cost of 1 sheet of Okume I can buy the wood for an entire boat! Good luck...