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The exhaust noise is almost gone while at idle -- I plan on insulating my engine cover next. It honestly didn't make as large of a difference inside the boat while surfing as I had hoped (though, I did have some leaks around the exhaust flange my first day out that I have since resolved.) I would say it cut the noise from behind the boat by around 50%. It was very easy to hear the driver talk while you were surfing -- I don't remember that ever being possible before.
My stern light does not get submerged @ idle or surf speed, but it does get hit with water all the time. I'd use some RTV or silicone to seal it up just to be safe.
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Idle is so quiet you can hear the flywheel turn. But it still rumbles! Surfing you can TALK not yell to the rider. Cruising fast you can hear the water against the hull.