PS-I have seen transom exhausts, above, at and below the waterline, side exhausts just in front of the transom, side like Mark's (although usually for generators), underwater exhausts flush to the hull, but I have never seen anything like the SeaRay one you posted. I guess I have never looked at a SeaRay out of the water. That is just weird.
All the rationale I have heard for underwater was reduction of blowback, sooting, noise etc. But then I have been told above the waterline is better because you can see the waterflow.