I believe the reason is the lack of protection of the supporting balsa wood in the deck. Water soaked through the cut at the screw-in port and seeped into the deck-core. I will probably remove part of the deck and replace the deck-core. I am still unsure if I shoud do this from the inside or from the outside.
Thanks for you reply
Le'a has the same issue. I plan to take aabout a a one by two foot section of the deck out this fall as soon as she comes out of the water. I can then clean up everything, replace the wood, put in lots of epoxy and then replace the deck section. I think the hard part will be finishing around the edge of the removed section. I have on of the solar powered vents and plan to put that inplce of the original port light. I think it was just a cheap inspection plate anyway. I could not find a replacement for the original as it appears to be a size not made anymore.
I have drilled small holes around the soft areas and then from the lowest hole injected epoxy into the area. Using the lowest hole insures that i will not get a "drain effect' and therby lets the liquid be forced into all the voids. I just did this a week ago and the deck is noticeably firmer now. make sure you tape the injection hole over after the treatment or you'll get a nice little piece of epoxy flow artwork on your deck.