My 88 H30 had the same issue when I hauled out last fall. The area was larger than just the head, reaching almost back to the black water thru hull. I haul every other winter, and hadn't seen this in the last 3 hauls. The survey didn't show any issues when I purchased the boat. Internet searches didn't show much, but some people believe that the area is under served by stringers, and became weak over time. Hitting it with a mallet didn't show much flexing, and I used a hole saw to cut in and get an idea of what the fiberglass looked like. All was sound, and I could barely move the area when pulling against the hole. I used a usb endoscope with my tablet to look around, and I never saw anything remarkable, except the large amount of space without support.
I thought about pulling up the head floor to get at the area to strengthen, but there is virtually no chance that I would be able to gain full, useful access to the entire area, and I was concerned about putting the floor back together and making it look good. The marina advised against going that direction, and thought that grinding down the area and building it back up with reinforcements from the outside would be the better bet, which is what I ended up having them do. $2k later, and a couple of weeks worth of work, and it looks and sounds like new. We will see how it looks when I haul again in Fall of '17
