Seattle,
My wife and I find the noise while running the unit to be acceptable. After a while, to me, it just becomes background. We don't run it at night, as for us, then it is not really needed. If you are a light sleeper, and the boat next door in a marina runs their's all night, it might be a bit bothersome.
One other point that has not been mentioned, is the heat generated by the exhaust. Not really a surprise (a furnace burning diesel having a hot exhaust), yet I have heard of a boat fire caused by the owner hanging a fender right over the furnace exhaust port. I don't remember the details, but do remember to check the area where the furnace exhausts to ensure there is nothing in the way. I also do not run it if rafted to another boat on that side of my boat. Just cautious, and wouldn't want to even discolour or leave soot on their boat. We have not experienced any soot build up on our boat.
A good friend has a Dickinson diesel fireplace on his boat and loves it. However, that unit requires space in the salon, and an exhaust port (chimney) above it. It does not use any ducting, just radiates the heat. Another option to consider.
Regards,
Tom