From your photos, your 1970 Boler floor appears to be constructed the same way as my former 1974 Boler 13. Its a solid fiberglass floor reinforced below with encased strips of wood. It has a textured gelcoat floor surface.
The difference is that your floor appears to have already been painted over, and partially on the bottom as well and possibly repaired before it was painted (?).
I believe your idea to epoxy plywood to the top surface would work. The application of a waterproof membrane is not necessary as epoxy is waterproof. Furthermore, I suggest you first sand the floor with 80 grit sandpaper to sand off the paint
and provide good adhesion for the epoxy.