on my raft and cat i've been using spar urethane varnish on top boards and polyurethane porch paint on bilge boards. this combo seems to be working best for me.
the spar is a little pricey but holds up well if not submerged and has a nice look too. after a few years it gets that honey gold color.
The one thing i would suggest is to epoxy the inside of the holes you make for your strapping and the outside edges of your plywood, too. whether you you paint, spar or whatever, those spots take the beatings and the epoxy holds up better and you can still varnish (or paint) over it.
down-side to the paint and spar: maintenance. you have to redo it sooner or later (depending on your usage and ab-usage & how pretty you want it to look