Go with the "spar" Urethane. It is better than using thompsons water seal, better than treated wood, just as good and cheaper than Marine Grade plywood, and a lot easier than encapsulating it in fiberglass resin (which is probably the best way, but a pain).
I used the Helmsman brand in a spray can. 3 coats, paying special attention to edges and where any cuts were made. You can then put some sort of non skid on or put carpet over it. I painted mine OD green and it worked great!