I donlt know these ribs at all, but a couple of questions:
if you knock your way along the flap, does it sound hollow, and / or does it significantly change note when compared to the Stbd side?
Also was the leak a drip or a gush?
I would guess looking at your pic that the flap is formed by a continuation of the layup of the bottom of the hull, in which case you may need to open it up, dig it out and replace the core.
There's guys on here do that sort of thing all the time, so I'll leave them to expand on that thought, but if you could let us know the P / S difference that might help.