Deck re-core from below

[sailingunity]

My 48 year old boat has had a somewhat inconsistent and pestering leak on the port side for a while now. As far as I can tell, the previous owner had issues with it as well and never could nail it down. I've rebedded hardware in the immediate area to no avail, as best I can guess the leak must be coming from somewhere "uphill" on the topside, and running horizontally inside the deck until it gets to an opening (which happens to be either above my workbench or in the aft cabin chain plate area).

Over the years, this has rotted out a good portion of the coring, as evidenced by the hollow sound of approx. 8 feet of side deck. There are no soft spots, per se, but my stanchions do move slightly as I walk down the deck.

My plan is to cut the skin out from below and remove the rotten coring, then see if I can pinpoint the leak. Reason for going in from below: I live in FL which gets rain almost daily, and this project will take me several weeks as I only have about 4 hours each Saturday to work on it.

I'm not quite settled on what to replace it with. I've read varied opinions on how how great / awful foams are, as well as how great / awful balsa is. Though I'm not super inclined to go with Balsa again, I can admit it has good physical properties and it's cheap. Alternatively, I thought about H80 + Coosa or G10 where reinforcement is needed (like the stanchions). It does seem like it would be easier to apply a board from the bottom than the mesh backed balsa.

Anyone care to change my mind?