A heating system is provided for heating fresh water on boats, marine vessels and the like. The boat's water supply is connected to a hot water tank, which in turn is in fluid communication with a heat exchanger. The coolant line for a boat engine passes through the heat exchanger, through the hot water tank and back to the engine. Heat from the engine is transferred from the coolant line to the water in the hot water tank at a rate that varies with engine operation. The heat exchanger allows additional heat transfer to the water as the coolant and water pass through the heat exchanger. A sensor regulates circulation of water through the heat exchanger. The heat exchanger may be retrofitted externally of an existing hot water tank by connecting the heat exchanger to existing water lines for the hot water tank and to an existing engine coolant line to effect heat transfer from the engine coolant line.