i've heard they make marine plywood but it is rather pricey and using it is not crucial. as for me, and most other posts i've seen, regular hardwood plywood and several coats of water sealer, especially on the ends. avoid what is called 'sheathing' as it is not designed for use as an outer layer, it's meant to have protection such as shingles or siding between it and the elements, and it usually has a rougher quality to it. my 2cents, birch hardwood plywood, 1/2 to 3/4" depending on the span between supports. i used 1/2 on mine as no span was more than 18-20".