Re: How much deck over the cabin do I remove?
OK these look like molded F/G pedestals that supported a dowel/tubular handrail.. not the usual style, and very difficult to replicate.. The coring looks very wet, the difficulty will be in determining how far it's extended. Have you done a 'tap test' around the area? That won't determine the dampness but will give you an idea how far any delamination has progressed.. a related issue.
Tap the deck with a plastic hammer or screwdriver handle somewhere central where no water has likely gotten to.. the hard 'tap' sound there is your control... as you tap towards the wet spots you'll hear a duller thud as you move over separated layers. You can map it out with a pencil to get an idea of the extent. Excessive delamination is a good sign of how far things have gone (at least)
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)