Before I bought the tractor I have I owned an Iseki with 2 cylinder diesel. I can't tell you exactly how many hours I could get from the little 6 gallon tank, but it was nothing short of amazing! I could use it all summer on little more than one tank of fuel, the engine was an Isuzu, rated at 16 hp. It would turn a 50" finnish mower or a 4' bush hog.
I've often thought of how well the engine would work in a marine application.