The rain and poor maintenence are probably the cause of the leak. But check these as well as they could leak and are common things that do. If it has a holding tank, chances are it has a thru hull fitting to dump tank overboard. Also one to draw in outside water for flushing, if you have moved those valves after a long time of no movement, they may drible water. Check the mast, too. When it rains in mine, the mast leaks. I have searched for a solution to that but none I have found fit how I would like to stop the leak in the mast without stepping it. Im in saltwater on the ICW in Titusville, FL and whenever there is water aboard and your in salt water, taste the water to see if its fresh or salty. Dont know if that helps or not but the best is to just grab a hose and spay the boat and if nothing comes in from the top, then it was poor maint. Hope that helps.