Ken.. You have a 22 ft boat that I suspect you are looking to use for recreation not racing or sailing across oceans.
The would lead me too believe the boat will be used moderately not pushed to the extreme.
You need to be the judge when to stop digging.
- The area too starboard (the bad ply) is it rotten, punky, crumbling separated from the fiberglass deck
- or is it wet but in contact with the fiberglass deck.
If it is structurally solid/together as plywood - not rotten dust, I would seek to leave it in place and once dry use a PENETRATING EPOXY SEALER like this at TotalBoat.
This will, when dry, bind the ply's together and provide the structure you need on that part of the deck without removing more of the deck skin.
In the area you have excavated, you will want to put a new piece of plywood. Attach the new piece to what is in the "hole" with some thickened epoxy. This will give you a solid base for your mast.
Then you will want rebuild the deck with fiberglass layers. Here you want to use a laminating resin as it will permit you to build the layers. Andy, at the link to BoatworksToday.com , demonstrated how to use laminating resin and glass mat & woven cloth to build the deck back up.
Once you complete the deck you will then drill holed for your bolts/screws At that point you will use the epoxy plug to make sure no water gets through the deck into the core.