This spot seemed a little less dry after sanding. It wasn't exactly wet. There was no drop of water there or anything. After laying up eight layers of fiberglass over this spot, I see water has intruded into the layers of my repair. You can see a white spot in the repair where the water is. This will never be a strong repair, and has to be ground back off before another repair can be attempted.
The floor above this spot has foam between the floor and hull, so moisture is leaking out of the foam, through the old repair and into the layers of fiberglass I just laid up. I made a hole in the floor and dug through the foam down to the hull, expecting to find waterlogged foam and water running out of it into my hole. I was surprised to find the foam felt only slightly moist, not real wet. I could not squeeze a drop of water out of any of the chunks I dug out of my little hole in the floor.
After leaving the hole open for a few days, I could no longer feel any moisture in the foam at the bottom of the hole. After cleaning the area with acetone, I mixed some polyester resin with a lot of hardener and painted over the area. When that hardened, it was wiped with acetone and another fiberglass repair was laid up. If there was any moisture left to weep out of that spot, then hopefully the fast curing resin sealed it in so the new repair won't be ruined by water intrusion.
If this didn't work, I would probably have excavated out all the foam in an area about a foot square above the repair and taken steps to keep the area directly above the repair dry.
I have been surprised by the foam they put in boats. It gets waterlogged pretty easily. Everyone I have asked said they see wet foam in all the boats they work on. Expanding urethane foam may seem like an easy way to fill an area and make the boat unsinkable, but water leaks into the foam and becomes trapped there. Eventually all that foam gets pretty heavy. Its not structural even if you imagine it is. Sometimes its easier to just leave it. When I cut the back of one pontoon off to make room for a boarding ladder, water literally poured out of the foam. After a week of being allowed to drain and being left open to the air, it was dry. I actually put fairing compound on it and laid fiberglass over that.