I have been a Webasto heater fan for my last six boats. I used the Webasto 3900 on my last boat, a Hunter 380 and it served me well. I now use the smaller model on my present Hunter 27 and it has done it usual excellent job.
The more recent Webastos are quiet both in operation and at the exhaust however, I would advise adding a muffler at the exit end of the exhaust line. Sure Marine in Seattle make them. It will make sure that no sound is heard on the docks when it is running.
I also had a small fuel filter installed on the fuel line leaving the fuel tank. They say it is not necessary but I did it anyway. It wash't a big cost.
I use my Webasto all winter long and have had no problems whatsoever. I could not live without my heating system so consider this a biased report. I have been using my present Webasto for four years with no problems. My previous 3900 on the 380 was used for nine years with one maintenance service during that time.