1/2" or 3/4" will both work. Just be sure that you NEVER use pressure treated wood on an aluminum boat as it will cause electrolytic pitting. High quality, exterior grade plywood sealed with 2-part epoxy, fiberglass mat and polyester resin, or several coats of Spar Urethane will seal the wood properly. Then mount it to the hull using only Stainless Steel hardware, preferably dipped in 3M 5200 marine sealant for good measure. That's really all there is to it. Good luck. Holler if you get stuck.