Abstract: An exhaust system for an internal combustion engine used in a marine propulsion system is provided with a water injection system by which water can be injected into the exhaust system. An engine control unit, which comprises a micro-processor, is used to select the rate of water injection into the exhaust system as a function of several predefined parameters. For example, engine speed and throttle position can be used by the micro-processor in the engine control unit to select a predefined rate of water flow into the exhaust system by selecting a predefined valve position, for an electronically controlled valve, that has been preselected and stored in the micro-processor.