Abstract: An exhaust system for a marine propulsion outboard drive wherein the exhaust gases are normally discharged to the atmosphere at a point below the level of the body of water in which the watercraft is operating. A catalyst bed is provided in the exhaust system and the catalyst bed is protected from damage by precluding the flow of water in the exhaust conduit to the catalyst bed in response to certain conditions. These conditions may be either rapid deceleration of the engine or watercraft, stopping of the engine, or any of a combination of sensed factors. The water is precluded either by purging it through air pressure or from generating heat in the exhaust conduit, by providing a heat source in the exhaust conduit that will cause water to vaporize and increase the pressure or by a valve in the exhaust conduit positioned below the catalytic bed. The preclusion of water is initiated for only a predetermined time or until the sensed condition no longer is existent.