[Captain Rich]

Check your vent line. Water may be splashing up the hull when you are running, getting under the clamshell fitting that covers the vent and running down the line into the tank. Put a larger clamshell on to protect the vent opening and put a drip loop in the vent line so water can't get a straight shot into the tank. Check your fill cap, if it is horizontal, can water sitting on it drip into the fill line? Check the top of the tank, is there a pinhole on the top of the tank which will allow standing water from clean-ups to drip into the tank ? I had a similar problem when I first got my boat, it turned out to be vent line issues. Good luck.