C&C used butyl tape to seal the hull to deck joint of most of their boats. This is an excellent construction detail. If you develop a leak at this joint, snug up the nut at the location of the leak. It is extremely important that you have a helper when doing this work. Have your helper hold a Phillips screwdriver on the machine screw in question to prevent it from spinning as you snug up the nut from below. If the screw is allowed to spin, it can break the seal and cause water to enter at that location. DO NOT ALLOW THE SCREW TO TURN. Do not tighten the nut too much as you do not want to squeeze out all the butyl tape from the joint. Tighten a little at a time and then go sailing or wash the deck and see what happens. If it still leaks, tighten some more. Just hope that a previous owner didn't over tighten those nuts in the past.