Here's what I would do...
Hi, David:If it were me, I would take the mast down, remove the maststep/hinge plate and its screw/bolt. I would then figure out how far out the sponginess radiates from that location. Hopefully, not too far. I would then get a 1/4 drill bit and a depth stop collar and measure from the bolt hole for the mast step the depth from the deck top to the inside/top of the headliner skin (i.e., how deep does the core go before it attaches to the lower fiberglass skin). Set your depth stop collar on the drill bit to an 1/8" shy of that measurement and drill a series of hole down through the deck in the spongy areas--about 1 inch apart. Your deck will look like swiss cheese when you're done. Then get a dental pick or stiff wire and start digging out all that probably rotten/wet core wood in between all of those new holes you just made. This will take the longest and be the hardest part of the whole repair. Take your time and get almost all of it out. Take a shop vac and suck out all the debris/dust as you go. Once you get it all dug out, tape off/mask off the perimeter of all the holes and get some West 105 epoxy, and fast hardener, mix it up with no additives. Pour/inject the clear mixed epoxy "soup" into each and every hole--you're not trying to fill them all the way. You just want to "wet out" the interior of all those holes and newly dug cavities. Let it settle for about 10 minutes or so. Then get some more epoxy and mix it up with some microfibers (strengthening/thickening agent) and some color additive (white makes it look like fiberglass)--mix this well and keep adding microfibers until it gets to the consistency of runny peanut butter. Pour/spoon/push the "peanut butter" into each hole until all filled. Keep pushing/adding a little more as it will settle/form air bubbles while it hardens. Put just enough in the holes to leave just a small bump over the level of the deck--when it hardens you can sand these bumps perfectly flush with the surrounding original deck level.Once hard and fair, you can paint the area--if you desire--or leave it natural until you're ready to paint the whole deck. The repaired area should now be solid as a rock. You can use this same method to repair any other spongy areas you find, inside and out, of the cabin.Good luck.Rich Lemmler, Jr.s/v TIME BANDIT1975 O'Day 22