I have the yam 15 2 stroke on a 3.4 zodiac. Usual days out are 4-5 hours mixed use, mid range revs to get on the plane & stay there. Fuel use varies 10 litres flat calm, 1 person, 15 litres 2 people, lumpy seas.
Don't sit on the fuel pipe like I did today & have the engine die just as you are crossing a busy channel!!! Filled my pants!!! ho hum you live & learn.