If you can find it, use MDO Plywood. It has a smooth face made from some kind of paper-like product. It is used for outdoor signage, and most of the raft trailer manufacturers around here (the rainy Northwest) use it. It seems like it would be slick, but really isn't in my experience. To finish it, I take high-quality marine varnish and thin it way down so it soaks deep into the wood, and doesn't leave a film. Either epoxy or lots of varnish on the exposed edges and you are good to go.