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Re: Help pinpointing a leak
The hole is called a "limber hole" and it is there to allow water to run past the stringers, or ribs, whichever you prefer to call them. The water should not be coming from "inside" the hole though. That stringer should be glassed into the hull at both sides and fairly well sealed.
Of course if you empty all tankage on the boat you will eliminate the question of a plumbing leak. C&C's are known for a "C&C smile", the keel is lightly attached compared to some makers and you can get a leak from the keel flexing, but I'd try to eliminate anything else before looking into that. (Either by diving to inspect, or by hauling to see if anything runs out the keel seam where you put water inside.)