Wayne -
Is the crack located down the keel a ways (i.e., where the sump ends and the lead begins) or is it really at the hull? The crack at the lead-sump joint is very common with older C&C's, and you can: (1) patch it up with some fairing putty, sand, and paint, but expect it to return next haulout; (2) check the keel boats...tighten...and then #1; or (3) loosen the keel bolts, have the yard lift you put for the day, pry the keel from the sump a couple of inches, clean, rebed with 5200 or a less permanent caulk, retighten and #1, except it won't return. The newest C&C's had this joint wraped with fiberglass and don't have this problem. Now, if it's really at the sump/hull joint, and its large, then look for cause inside structurally. It's probably minor (e.g., got set down too fast on the stands) and the first thing I'd do is dig it out a bit, fair with epoxy and colloidal silica (very strong), seal it up with epoxy barrier coat, paint. If it's not severe (structural), it won't return.
Good luck, Greg