Re: oh what to do with a leaky hull-deck cavity?
Well the boat is a Columbia29 mkii. It has a classical inward flange rather than the H-joint from more recent columbias. That cavity you see is between the flange (with the layup visible) and the bottom of the deck sandwich with can be seen on the top side of the pic.
As you can see in the picture I cut an opening in the tabbing inside the boat (I since then reglassed it because I cannot sail with that open, when I tried I made a pond of my quarterberth (cushions were out thankfully).