Here's what I did!
Thanks everyone for the replies!Based on your reccomendations, this is what I ended up doing.I removed the water fill inlet fitting from the deck. I found that there was no bond between the deck surface on the bedding compound under the fitting. I also found saturated plywood between the outer hull glass and the inner hull. I was able to remove most of the wet and rotted wood by using a 12" long sawzall blade with tape wrapped around for a handle and bent slightly. I worked this in between the fiberglass skins by hand through the deck fill hole, for about 6" - 8" all around the fitting, removing as much of the rotted plywood as possible. I then blew air into the void using the exhaust end of a shop vac for about 3 hours. This seemed to remove most of the moisture that was left. I then made a marine plywood reinforcing plate that fit below the deck, and one that would be placed above temporarily, and bolted through the fitting hole and the plate below. I was cafefull to put masking tape down on the topside all around the area to prevent scratching and also to keep the urethane sealant from bonding to the surface. I then sprayed meduim expansion urethane foam into the cavity through a small hole drilled in the top plate. I allowed the foam to cure the stated time, scaped off a little overage and removed the bolt holding the top plate down. The foam had filled the void entirely. I then drilled a hole through the foam and bottom plate, and re-installed the deck fill fitting. This time I through bolted the fitting, and used sikaflex sealant, leaving the sealant to set for a few days before I snug up the through bolts. I then drilled some 3/8" holes through the inside layer of glass, carefull to only go into the wood and not all the way through using a drill stop to hold the depth to 1/2". I did this to see how far the rot had gone around the fitting, and also how far the foam had penetrated. I found that the foam had filled the voids quite thoroughly, but did find some more rot a few inches past the foam. I plan to leave the holes open on the inside to allow the core to dry, and will do the same process only from below this time, by drilling a few 2" or 3" holes through the glass below, removing the rotted wood, allowing it to dry, and spraying foam into the void in a similar manner. This time I will also install a marine plywood reinforcing plate below, but will brace it from below with screws into the sound core, and blocking wedged underneath to the v-berth. I found the deck to be extremely solid feeling after the first repair, without any noticeable bulge or damage. The marine plywood plate below will be finished to match the interior cabinetry, and will be hardly noticeable.Thanks for all your input!!