Got a chance to go down to the boat Saturday. Aasem was right - it was diesel
fuel. I had filled the tank 6-7 weeks ago and the tank design allowed the
fuel to press up against the point that the sender is installed. The gasket seal there was not complete and diesel slowly seeped out - there was still a pool of it in a depressed area on the top of the tank (plastic tank). I pumped the bilge and some of the diesel out of the tank in addition to putting a sealer on the gasket. Lesson learned - next time I won't completely fill the tank!
Thanks for the input/comments.
Drew