A heat exchange system for use on an engine-powered watercraft includes a liquid cooling system for cooling the engine using a first heat exchanger and a water heating system using a second heat exchanger for heating water. Raw water from an external water source is passed through each heat exchanger. Water used to cool the engine coolant inside the first heat exchanger exits the watercraft. Water heated by the second heat exchanger is passed to either an intake conduit or at least one onboard accessory system for flushing thereof to kill aquatic invasive species. A valve inside the second heat exchanger opens to release heated water when the heated water reaches a temperature of at least 140° F. Heated coolant from the first heat exchanger passes through the second heat exchanger to transfer heat to the water inside the second heat exchanger.