actually that's not true - marked gas has been around long before ethanol was ever thought of. Marked gas is simply a fuel used in an off-road application - be that diesel in a farmers tractor, a boat, or even a lawnmower - since it's not being used on the roads, most road based taxes are excluded. It's "marked" by adding a dye - theoretically if there is suspicion you are illegally using it on the road, an LEO can dip the tank and if it's coloured, then hefty fines are given. As it's been pointed out, in the marine world, it's typically just a mid-grade gas.