I second Gunni. I'd use either okoumi or meranti BS1088 ply. 1088 is A/B face grade, BS6566 is B/C face plies. BS rated plywood specifies more plies than average fir "marine" plywood available from your local lumberyard, and boil proof glue. Plywood rating systems are all over the place. I was able to buy submersible "marine" ply from my local lumberyard (they call it "marine ply" because it was immersion rated glue), but it was fir, and didn't have as many plies per thickness as a BS graded ply. Fir is less desirable because it will crack and check, even if painted with epoxy resin.
Okoume is relatively light, not terribly rot resistant, but certainly ok for interior use, and used in many hull building projects where encased in epoxy and glass. Meranti can be good too, but I understand it's more brittle and prone to cracking while being worked. Sapele is expensive, but has some beautiful grain patterns, if you had something that you wanted to look especially nice. Chesapeake Light Craft uses okoume for hulls and bulkheads in their kayak kits, and sapele for decks.
You can order BS graded plywood from Noah's Marine, and they will ship full panels, 1/2, 1/4, and possibly down to 1/8 size panels.
http://www.noahsmarine.com/index.asp
