Best fuel efficiency in my boat at cruising speed (around 3000 RPM) is about 8.3 gallons per hour. At W.O.T. (AROUND 5800 rpm) that drastically decreases to about 26 gallons per hour, but what fun!!
Saving gas while boating all depends on what you are trying to accomplish.
If you want more hours on the water with less gas being used simply throttle down. If you are looking to economically get from point A to point B you need to find the proper cruising speed for your boat and motor.
Having the right prop can make a world of difference in gas savings at cruising speed. Your motor should be propped so at normal load your required / recommended W.O.T. RPM's are attained. I have tried many props to ensure I am getting everything I can out of my boat and feel it has been well worth the effort.
Dan