I've had excellent luck using a "drill and fill" technique for soft decks on my 1974 Catalina 22. It's important to dry the deck core completely first- I was lucky and mine was already dried out. I've heard you can dry it out with many small holes, and displace some of the water with a solvent, like acetone.
On mine, I drilled a few holes, and glued film canisters over them with a hole drilled in the bottom- as funnels. I kept these full with unthickened west system epoxy, and pushed on the deck with my hands to "pump" the epoxy between the fiberglass and the wood core. After hardening, I removed the film canisters, chamfered the holes with a drill bit, and filled with gel-coat patch. It's been one year, and these parts of the deck feel more solid than the rest of the boat still.