Most boats have fairing filler at the hull to keel joint. The keel is usually just a bit smaller than the hull, and filler makes the transition smooth. As the filler ages it will tend to crack at this joint. Many manufacturers us inferior filler for this purpose as well, in many cases Bondo. You can grind out the crack and refill it with epoxy thickened with microballoons for a 1 or 2 year fix. The only way to fix it right is going to be removing the old filler and refairing the joint with epoxy and microballoons. It's a bigger job than it sounds like, but you won't have to mess with it again for quite a few years.