http://www.aitwood.com/
They're in Anaheim. I'm also tempted to use the curved sections, but maybe making too much work...
My dad said the the seat bottom is what really gets soaked, foam against wood. Marine plywood wouldn't matter on the sides and isn't enough on the bottoms. A composite of some sort might be cool.
Of course this is easy to talk about. When I get the fiberglass/gel issues dealt with and a moter in place I'll get more concerned about my seats.