I would use your favorite faced plywood (3/4" thick). It comes faced in maple, cherry, oak, birch, etc. Plywood is far more stable than solid wood and also much cheaper.
Then, poly would work good but I would go with a spar-varnish (technically, I think it's still a polyurethane as well). Spar-varnish was developed for the old wooden boats and we all know what water they sailed on, right?
dual 175 MH
w/ dual 32W PC