Lead that's been submerged, especially in deep water, may contain pockets of water, even after being out of the water for a long period of time. Drop one of those in a pot of molten lead and you're likely to get an explosion. You may be able to dry them out well enough by heating them in an oven for ah hour or so, but even then you're taking a chance. I'd stay away from them.
A keel from a sailboat would weigh several hundred pounds. Let me know when you are going to put that hunk of lead in a bullet melting pot...I want to watch.