I have Centek water-lift mufflers on my twin 160hp turbocharged Yanmars. The exhaust sound is audible but not at all intrusive, even at WOT. I am more comfortable with my exhausts discharging above the water line - one less thing to worry about. My first check on starting engines and generator is a quick trip to the stern rail to listen for that reassuring burble-burble-whooosh sound of water coming out along with the exhaust. Seems to me that this check would not be possible with underwater exhausts.
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If it ain't broke, don't fix it