Abstract: A marine fuel supply system is disclosed in which a salt water sensor is positioned between a fuel reservoir and a pump used to pump fuel from the reservoir to an internal combustion engine. The water sensor is located near to the bottom of a closed vessel forming part of a water separator so as to sense when salt water separated from the fuel has reached a pre-determined level. A signal is then sent to indicator means in the form of an electric lamp and buzzer to warn an operator of the marine craft of the need to drain off the collected salt water.