My understanding is that there has to has to be a low pressure/no pressure escape path for the exhaust that comes out of the back of the outboard. In other words, if the ports are plugged, you can ruin the motor.

My problem I had earlier is that when I put a rubber "gasket" around the back of the motor and that lead exhaust down to the water out the bottom of the well, there was too much back pressure when the boat was under power and the stern/gasket dipped into the wake.

What I have done now (with my Yamaha) is to build an aluminum/fiberglass "cup" around the exhaust ports at the back of the engine, and connect a neoprene hose to carry those fumes out a small hole in the transom. After almost 900 miles of cruising this year with the motor operating at full blast about half the time (sailing on the big ponds here - Great Lakes - involves a lot of motoring and, when beating in particular, motor sailing. So, I recommend the same resolution to others.