Re: Building seat frames......
1/2" exterior grade plywood good 1 side, sealed with an oil base sealer, then you can paint of use contact cement and cover with vinyl. if you keep the boat covered when not in use should last as long as the upolstery of longer. it should give you atleast 10 years of service. the trick is sealing the wood. coat all your edges before assembly. 45 years in the general contracting and remodel business, boating experience.
there are some new foam core materials, but they have'nt proven themselves. as far as holding fasteners. they want you to use special glue.
starboard is heavy, expensive.
also if your deck is carpeted, a spacer between deck and the box, will really extend the life. the carpet hold water, especially, where things are attached. it's the wick theory. wet is going to wick to the dry, then get trapped.