plywood cored GRP laminate

[EricT]

I am about to build a modular Polyester glass hulled catamaran houseboat, and because the sides and bottom of each module (6ft long x 3 x 3) are flat I am considering using half inch ply as a core. Some of the modules will be used as tanks for fresh, black and graywater so will be exposed to moisture from inside and out. It is my intention to coat the inside and outside of the hulls (below the waterline) with epoxy.
Will this be sufficient to prevent moisture getting to the ply and causing rot or delamination, or both?
Cost is a consideration here but so is longevity.
The boat will be used in saltwater.
Any ideas?

[Raggi_Thor]

Normally, you don't want to combine polyester and epoxy. With plywood you should use only epoxy and no polyester. You can add a thin layer of glass as a protection to avoid scratches, but you probably don't need the glass for strengt (stiffness).

While epoxy costs many times more than polyester, the total cost may be lower because you use more plywood and less glass (and thus less resin).