wood coring question

[wellcraftv20step]

I noticed the 89x27 fuel tank hatch was a bit soft on a corner, so I lifted out the hatch and of course it was soaked and the glass mat was separated from the plywood ! what is the most common material to recore it with today. I was just looking at the Depot website and wondering if the dryply would work since if its laminated properly their should be no water intrusion. the caulking was poorly done and the 2 inspection covers were also not properly fastened.one other thing was that the plywood was not beveled at the edges ,it sat flat against the lip of the hatch which is why the mat on top never adhered to the one inch lip . please give me your thoughts