Unfortunately, I have some experience with this issue...

I was mounting a set of stainless portapotty hold down brackets in the V of the V Berth area of my 23' Venture Sailboat one Sunday morning at the Ft Belvoir Marina. Was alone of course. Drilled the aft two holes and screwed it down just fine (this boat has an inner liner) but when I drilled the port forward hole, I got a little fountain of water about 18" high. Of course the boat was in the water!
Like the good little Dutch Boy, I quickly put my finger in the dike and quelled the onslaught of water. What to do? No one else around, nothing to plug the hole with, no caulk or anything. By the time I could have gotten some, the boat would have had a lot of water in it....
So, like a good engineer, I proceeded to put the screw in the hole, it was a tight fit. Drilled the other hole, more water and one more screw. It lasted all season and when I hauled the boat in the fall, saw two lilttle tips protruding from the bottom.
Removed the screws, cut them off, filled the holes with something or other, I think 3M may only have been 2 1/2 M at the time, and reinserted them. No more problems. Have not told that story very often....Wonder why? :shock: :cry
Charlie