rotted plywood-need help

[stripes]

Guys the wood laminate in my old Monark has rotted to the point that it now resembles mulch. Is there something that I can use besides plywood to reform the laminate? I replaced it once about 10 years ago using 1/4" plywood and fiberglass resin/cloth. It was a PITA and it was expensive even by DIY standards. It must have had a pinhole that I missed cause water has migrated all over the boat in that plywood between the outer hull and the inner floor. I can easily access all parts of the hull since its a tri-hull, but if I go to this kind of time and expense again, I would like to use a composite rather than plywood if there is such a thing that is comparably priced. This is a 1980 Monark and most would say scrap it if they saw it, but it has significant sentimental value so i thought that I would throw this out there. Thanks in advance for any/all replies.