How to avoid warping?

About 5 years ago I added a small front deck and a rear seat to my bare bones 1436 jon boat. I used plywood coated with several layers of Helmsman poly, then carpeted. The front deck is still solid, but the wood under the rear seat is buckling and warping, especially around the stainless screws holding it down. Last night I removed the wood under the rear seat with plans of replacing it, but I noticed a lot of moisture trapped between the wood and the bench. It hasn't rained here in days.

This moisture is obviously causing the problem, but how do I keep it out? Any help is appreciated, as I'd prefer not to have to replace this every 5 years.

The boat does sit outside, but is covered.



P.S. This is my first post but I've been reading this site for weeks. What a great resource you guys have created!

Something I've been thinking about doing with mine: Run a fairly heavy bead of exterior caulking around the perimeter of the bench, then lay your carpeted plywood on top of it. Make sure the caulking is in direct contact with the plywood, though, not the carpet. My thinking is that it should keep the water from getting under the plywood where it can't evaporate.

That is a very good idea. I think I'm also going to leave about a 1 inch lip over the front and back edge to keep the water out better in the first place.