Raritan 20 Gal Water Heater A combination of Raritan's modern polymer jacket and efficient insulating foam and glass-lined tank provides a superior marine water heater value. Features include ignition-protected thermostat and an innovative heat exchanger is over seven feet in length and is evenly coiled throughout the tank providing hot water rapidly and very efficiently. These water heaters are designed to withstand the rigors of the harsh marine environment and tank is protected by a 75-PSI temperature and pressure relief valve.