When mixed with gasoline, ethanol is an oxygenator and pollution inhibitor that slightly reduces fuel economy. It took the place of a previous additive that filled the same role and worked well, with one exception: It was a ground-water contaminant. But ethanol, also called gasohol, introduced a steep learning curve for boat owners. It can have a powerful, solvent-like effect on a variety of surfaces, especially those of hoses, gaskets, and fiberglass tanks. It also scours tanks, carburetors, and fuel-injection systems, removing deposits but clogging jets, orifices, and filters.