Plastic inspection ports with snap-in covers. The flimsy plastic frame distorts and is allowing water to leak onto the aluminum fuel tank. Tests also show that the covers often leak badly. The cut out for the engine cables at upper right is below the level of the transom. When the well fills up with water, it partially drains into the hull.
Many people make the mistake of thinking that the rubber boot around the cables keep the water out.
And the well drain holes often plug up with debris like leaves and pieces of paper. Oh, those wonderful cheap, flimsy, plastic inspection ports. Like most boats this size, the deck is full of them, and the deck flexes, meaning that the inspection ports bend, distort, and do not seal. Unfortunately, the aluminum fuel tank is under one of them, and of course water was leaking onto the fuel tank. But since we couldn't see the tank, we have no idea of what is happening to it.