Foam core seems to be deteriorated

I am working on a delaminated section of deck between the pylons of my Hobie 16. This is my first boat and I just bought it, so I don't know very much history of the boat. Upon drilling the holes to repair the delamination, I noticed the foam is creamy white in the holes closest to the edge of the delaminated section, and nearly black as I get inside the repair area. My concern is that I will be using a lot of epoxy to repair this section, as I will likely be filling the void than gluing the foam back to the fiberglass.
Should I change techniques and use a low density filler in the epoxy to fill the gap? My concern with this is the thickness of the resin and will it be an effective repair. Or, should I use a thin cutting wheel on my Dremel tool and remove the outer fiberglass, remove and replace the bad foam, glue and/or glass the cut out section? If I go this route, where do I purchase the correct foam?
On the other pontoon, there is a hatch installed just forward of the front pylon. I would like to remove the hatch and rebuild that section of the deck, as the boat did not originally come with a hatch and this is the weakest part of the boat. Should I stop worrying about the hatch?

Thank you!

This might be the better route if the foam is deteroriated. I recently did a similar repair on a windsurf board where the inner core had deteriorated and failed which then lead to the outer sandwich/skin failing. I routed out the deck around the entire soft area, cleaned up the core material, then poured in 8lb density expanding urethane foam from uscomposites.com (this is similar to the stuff you can get in a can at Home Depot, but super strong and MUCH higher quality). Sanded the foam fair then fiberglassed over it. Now the board is solid.

In any case, if a large area of the foam is shot, it's probably better to replace the foam than to just fill the whole void with epoxy (heavy!).

sm