Purchase a bottle or two of Heet or another isopropyl-alcohol based fuel treatment. These products usually advertise themselves as preventing fuel-line freeze-ups, because they remove the water that can freeze inside a fuel line. Sixteen ounces will be good for about 25 gallons of gasoline or diesel.
Pour into your gas tank the recommended amount. Follow manufacturer's directions and drive your vehicle, which will allow the product to flow into all the areas of your fuel system. At every fill afterwards, pour in a maintenance dose of 8 ounces.
Keep the tank filled to prevent water build-up in your fuel line. Most modern fuels have greater water solubility and may be able to handle small amounts of water. Larger quantities of water may need to be addressed by a professional mechanic.