deck repair

I've searched the web far and wide, including C&C forum on this site, and can't find anything regarding deck under/around bow hatch of a C&C 24... 1983 C&C 24 leaking hatch: Plexi/lexan is fine, rubber seals are iffy but hatch bedding is toast. Removed the whole assembly to find the attached...

I know it needs a lot of work and not quite sure of the scope or what the best options are. I want to do it right, I am in no hurry and will do the work myself. Btw the deck is not soft, and doesnt appear to be leaking anywhere. The cabin does not smell like water damage. Thanks!

I have researched this topic multiple times, so I felt comfortable enough to cut into the deck of my O'day 22 to begin replacing a wet core beneath the mast. I first cut out about a 12" x 12" area immediately surrounding the mast. I expected the core to be wet and weak, but this core was POWDER...