Fill up at the Gas Station, before hitting the water. The price at the pumps on the water is always going to be more than on land.
Drive Smoother. The smoother you accelerate and decelerate the less gas you are going to burn.
Slow down.
Minimize idle times
Ensure that your engine is well tuned.
Know where you are going.
Minimize use of onboard generators.
Proper use of the trim reduces drag, especially when accelerating to plaining speeds.
Make sure that you have the right prop on your boat, the wrong prop will be slow out of the hole or lack top end speed therefore making it inefficient.
Lose weight. Unnecessary items will weigh you down and in turn burn more fuel.