Help, mines doing the same. Our boat is 7 months old, the heating turns itself off after an hour, then if you re-start it, there's smoke billowing out of the exhaust. Our diesel tank is full so its not condensation, but we did have a diesel leak when we bought the boat and the company fixed it on the cheap!! Its really annoying, constantly listening for it to go off so you can start it again. Not a problem in summer, but the sqirrel can only do so much on a 65ft boat!
The engineers are coming next week to look at it. I'll let you know what they say