Raritan 12-Gallon Water Heater w/o Heat Exchanger - 240V [171202]12 Gallon Water Heater without Heat Exchanger 240V A marine water heater is simply a small, insulated tank downstream of the pump. You must have a pressurized water system to operate a water heater. The pump draws water from the storage tank(s) and fills the water heater tank. Inside the water heater is an electrical heating element and usually a coiled tube called a heat exchanger. When AC power is available, the electrical element (controlled by a