The way I get maximum moisture protection for solid wood or plywood is a two part system. First I seal it with a penentrating epoxy, I use something called CPES. Then because epoxy isn't UV stable, I top coat it with a good exterior/marine varnish. I use Epifanes marine varnish or HybriVar by Target Coatings.
I use this method for outdoor furnish, plywood kayaks, trailer boxes, anything wood outside that I want to protect.
For the plywood base of a RTT that doesn't get a lot of direct moisture/rain, a few coats of a good exterior water based poly like HybriVar should be enough.