How to fix soft spots

[SaltyKayak]

I have recently purchased a 10 year old catamaran style boat. I have noticed there are 2 soft spots on the deck. One I found when I removed an access panel and water had eaten into the plywood all around the edges and is still wet. The other is in the side of the floor and I would need to drill a hole to get at it.

What do I need to do to fill in the missing and crumbling plywood (filler?) and how do I fill the soft spot in the floor?

thanx, SaltyKayak