You might get 2.5 MPG at 3000-3500 RPM. Much less at high speed and while wakeboarding. Less if the boat is loaded. That's just based on averages for that size of inboard. An owner might give you their numbers.
When I'm riding I run with about 2,600-3k lbs of balast depending on whether I'm boarding or surfing, but I just want to know the fuel range fairly empty and then I can decide how much I will ride based on distance between fuel stops.