Sounds like your boat is a cat? I have never sailed on a cat, but does the other hull have the same access cap. Maybe check it out to see if it has an O-ring. If not I was contemplating something similar on my day sailor and found new ones in most marine catalogs.
In addition, my brother, a friend, and I almost sunk a JY-14 when we turtled after test sailing it for a yacht club that was looking to buy a fleet of them. The problem was a small crack in the fiberglass near the deck hull joint at the bow. When the boat turtled, water was able to leak into the void (which was intended to keep the boat afloat in situations like this) and weigh the boat down.
Take a good look for other spots water may be leaking into the hull. I am serious when I say, the crack looked like a 2 inch scratch from a trailer, but when we poured the water back out of it, it amazed us all.