Actually, I'm not sure I would t say that. If that, let's call it a lip, is sealed together (fiberglassed) on the bottom (where your arrow is pointed,) then yes, it could restrict the downward movement of water. If not, then water would be able to intrude into the lower section of the hull. Fingers crossed here. I can't tell if it is actually glassed together or not.
I do know that the core is compromised aft of the opening above the bottom curve. I am going to backfill this area as well as the top where the coring is rotted.
The hull sounds solid and normal below the arrow, so I am going to repair the area around the vent and leave the lower hull alone. It appears that the only way to access the lower hull core would be to cut away the inside skin. Not interested in going there unless there is an obvious problem. It appears that this is a fix that will keep anything else that may be going on from getting worse. Time will tell.
In the first and second photo, the area that is gray is the inner-skin and the fiberglass backing that gives the vent the finished look is screwed to the inner-skin. Where the bottom of the backing meets the inner-skin, the caulking is compromised. Going to clean that out loosen the screws and recaulk with a good marine grade adhesive caulk (thinking lifecaulk or 4000 UV). I think the water that is finding it's way into the cabin is from this poor caulk seal.
“Only the mountain has lived long enough to listen objectively to the howl of the wolf.”
― Aldo Leopold