In many cases, it is a result of Catalina, in a less than wise move, using plywood in the bilge for the keel support. I believe it was anohter member here, Maine Sail, who posted some very disturbing photos of what happens to plywood in a bilge.
A proper fix involves dropping the keel, removing the rotted plywood and rebuilding the keel support area with a very thick layer of fiberglass, and then rebedding the keel.
Filling the crack with filler compound is going to look like a repair, but it really isn't—it's just a cosmetic cover up of the problem. Without rebuilding the keel support inside the hull, rebedding the keel and torquing the keel bolts to the proper specification, you run the risk of losing the keel.
In all fairness, I should also mention this smile is typical and normal for Catalinas and never seem to worry about it which always seemed strange to me being a problem unique to Catalina. If another boater with a different manufacturer boat noticed this problem, I would expect they would be very concerned. I could never differentiate the mentality.