You should always empty the gas from your boat's tank when you're getting ready to store it for the season. It's dangerous to leave flammable liquid sitting, and the gas can lose its combustibility by the beginning of the next boating season. For your money and piece of mind, it's best to get that gas out of the tank and into another engine that can use it.
Connect your fuel line hoses to either end of the fuel line primer bulb. (Fuel line hoses 5/16-inch or 3/8-inch will fit on both ends of the bulb.)
Remove the fuel cap from your boat's gas tank and place one end of the hose in, going as deep as needed to access the gasoline.
Place the gas tank (or any other approved receptacle) lower than your boat gas tank and put the other end of the fuel hose inside the opening.
Pump the primer bulb to begin the flow from boat gas tank to gas can/receptacle.