Surely if there is water leaking in from the keel box, it can only mean the hull to keel box integrity has been compromised and cracked, sounds to me like you need someone qualified to look at it, if it is still in seawater the easiest way to tell where it has come from is by tasting to see if it is fresh or salt water.
My case is a lot different to yours, sorry i got a bit confused, my boat just has a bit of the filler cracking away from the slab and is purely cosmetic.
Hope you get it sorted