We had a center cockpit boat with engine exhaust going out port side not far above the waterline.
Of course, boat is loaded heavily for cruising and exhaust was closer to the waterline.
We occasionally got exhaust odour in the cockpit when wind, etc. was just right, but one time the boat got swatted by a big wave (engine not running, heeled opposite side) just right and water was forced back up the exhaust elbow
into the engine. We were on passage
, running the engine every day and with use of decompression levers, water was forced back out, engine eventually started and no permanent damage.
At the next anchorage I puzzled over what to do. The exhaust outlet had a pipe stub sticking out the side of the hull about 1/2" so I cut a piece of old yellow foulweather gear
material to a rectangle, wrapped the end around the pipe stub, about 1 1/2 turns and hose clamped it in place with about 8" of it flopping free. Thus when sailing it and the engine off it flattened against the hull and acted as a flap valve and closed.
When running the engine the exhaust water and fumes were transferred directly into the water and seemed quieter and less fumes blown into the cockpit. It would get barnacles
and such growth on it but when sailing it seemed to get cleaned off again.
We sailed many thousands of miles with that in place and sold
the boat with it still in place. A very low-tech solution. The only problem was that people used to shout that something was dragging in the water off the side of our boat.