Interesting subject, and it just so happens that I'm in the middle of writing an article for the MainBrace concerning the re bedding of the mast step on the MK-II's, but the construction method used on all the versions of the C-22 are very similar. Yes there are voids between the deck and the liner. As already been discussed, the factory placed plywood between the cabin liner and the cabin/deck. When ever I drill a new hole in the cabin top or deck, or when ever I remove a piece of hardware, I use my Dremel tool and undercut the plywood core. The factory bonded these parts together with what appeared to be globs of thickened resin, and on occasion, this is what I find between the liner and the cabin or deck. This resin mixture was evidently applied in a haphazard way, which lead to voids. When I removed the mast step several years ago to rebed it, the wood was bone dry. So after I undercut the coring in the mounting holes, and also the mast wiring penetration, (which was relatively close by), I taped over the holes in the cabin liner, and using warmed WEST epoxy slightly thickened with high strength powder, I started pouring the epoxy resin into the holes. The warmed resin flows much better, and will be absorbed into the wood better. I ended up pouring over a quart of resin to fill three little holes, and the mast wiring penetration, so never knowing what I'll encounter, I always have a good supply of WEST epoxy on hand. I could feel the cabin top and portions of the cabin front get warm as the epoxy kicked off inside. So the voids are now indiscriminately filled with solid epoxy which strengthens the boat even further. After the epoxy cured, I redrilled the holes, and now "if" water ever leaked past the hardware, it won't penetrate into the wood coring. The WEST epoxy website has great information on fiberglass repair, and re bedding hardware on fiberglass boats.
Don