Out of curiosity last year, on a quiet day at the marina early in the season (not many boats in the water yet) I did some testing with my generator. With it running under a moderate load, most of the noise outside is the hum of the bilge blowers, not noise from the generator. I'm currently collecting pieces to re-mount the blowers with better vibration isolation, as most of the noise is the hull sides resonating, not the airflow.
The actual generator exhaust is very quiet. Once you're about 50 feet away, you can't hear the exhaust at all, and even at 30 feet it's very quiet. At 50 feet, you just hear some water pouring / splashing out (it's not quite a steady stream, but it's not overly splashy and it's pretty close to the waterline, so it's not very loud and sounds fairly natural). On the non-exhaust side, you hear nothing but the blowers unless you're standing within 5 feet of the boat. The blower noise carries the furthest at a good 100 - 200 feet, but by 100 feet it's just a faint hum. If only it were that quiet inside the boat (not much engine room sound insulation and no sound shield on the generator).