Lots of possibilities. Overhead leaks can come from the screws holding the floor panel in the cockpit. The fixed side ports can leak as well. The fiberglass floor that the aft cabin mattress sits on actually continues aft under the wooden aft bulkhead and forms the floor support and pan for the holding tank to starboard and the diesel fuel tank to port. There are many sources of leaks from items attached to the cockpit coaming - arch support (especially the screws), stern rail, waste pump-out, fuel fill, vents, shore power connector, etc. When any of these things leak (and they always do), the water flows down into the lazarette on either side, then down around one of the tanks and then under the bulkhead and your mattress. You can look for water accumulating in the lazarette and see if there is any water staining on the underside of the wooden panels that cover either of the aft tanks. Good Luck!