if you're sure there's no water in it, dump 5 gallons in your car before you refill it. Usually what I do when I have gas sitting around. If it's going through a water seperator on its way out of the boat, that may be why you're seeing no water. If you want to run the tank out, seafoam it afterwards to help clean it out a bit. I'd change the oil in the tank too. Over time moisture gets to the bottom and mucks up. you dont want that running through your engine. It's only about a gallon of oil. lower unit also if you hadn't done that already