Boats don't get MPG, they get HPG (hours per gallon). I believe a V8 would be turning less rpm's at cruising speed than a 6 (given they're both installed in a boat of the same weight). The 6 would be huffing to push the load, while the 8 would be purring. I had a 26' Cigarette with a 454. As long as I didn't run it WOT all day, I could cruise several weekends on one (90 gal.) tank.
That being said, when someone says "lake boat", this is all that comes to mind:
