Could be from a lot of places,A leaky thru-hull, a fresh water line leak, a loose deck fitting, the list goes on. Simple rain water could be getting in someplace, Even seen a few that had a loose keel, and water would seep in from below. All you can do is dry it up, and look for any traces of water leaks. And make sure your bilge pump is working properly. I personally haven't seen to many boats that did not have some water there.