Most likely coolant, if your boat uses coolant; The only way I can think of fuel getting in there is if you have a fuel cooler. My advice is to stop using the boat until you fix this.
There is a likely heat exchanger dying somewhere. Have you checked your coolant levels? have you checked the color of the coolant in the tank?
"There's the Right Way, the Wrong Way, and what some guy says he's gotten away with"