In order to clean your boat fuel tank, you must first siphon out the gas that is left inside. You need a large bucket, preferably a five gallon one. Make sure you use a clear hose and not a water hose. You will not be able to see the gas. Push the hose all the way into the tank below the gas level. Do not siphon gas with your mouth. This is dangerous. Use a primer bulb instead. Make sure when the gas comes out, you put it in the bucket. Do not spill any on the ground.