I had a similar issue on my starboard hull. I cut the skin off from the outside, being careful to keep the skin intact as I peeled it off. I then replaced the balsa core with foam core, and reapplied the skin. Then, faired the cuts and glassed over to seal it all up. Worked quite well.

My issue was from the back of the keel. There was a tiny crack at the back of the keel where it meets the hull. This crack went through the solid glass in the middle of the boat to the starboard side where the core was. This was the source of my leak. I'm not sure the screws by the deck are the issue on yours. I think you would see wet core down the side of the hull as well, which doesn't appear to be the case. Plus, the hull where the screws go through up there is solid. The core basically stops at the outside edge. There is then a flange of solid glass that the deck and toe rail screw into. At least, that's how it was on my boat.