My old boat used to have a super charger on it and would go through about 9-10 gallons per hour
It was really frustrating, we went to lake powell and it was just rediculous, we put in 12 gallons of gas (it wasn't even empty when we put it in) to get back to the harbor (20 miles away) and ran out of gas 10 miles out.
However with my new boat after we serviced everything and got a valve job I've been out for three hours before and only used 6 gallons! Thats with 6 people and 1800 pounds. Needless to say I love this boat.
I have found that the best way to be safe and fair for gas costs is just to fill it every single time we finish. Makes life simple and avoids the whole "Here's three bucks, thanks for the day on the lake."