Taller and heavier boats will affect you gas mileage exponentially in my experience. I can get about 22 towing the S&S at 65 with 3 and minimal equipment. Load it up with stuff, 5 people, full cooler, toys, full gas vs 1/4 tank...
Pretty soon I'm looking at 16.
I spent most of my money on booze, broads and boats. The rest I wasted. - Elmore Leonard