Took a boat tour while I was in Berlin along the river in December and thought the cruise was ok. The boat was heated and it had the glass door along the side and top roof, so one could see places from as many angles as possible.
The tour was just one hour, so it was really a quick 2-time turn around once the tour started. Overall, it gave different angles of how one could see Berlin, but I felt like there could be more information/interesting facts and history (rather than just simply pointing out what each buildings was all about).
Overall, a decent way to spend time in a different way, but I wouldn't say it is a must do either. It is best as a supplementary information and probably doing other tours you may get more information you need if you wanted to learn more about Berlin.
The other thing was....because of the heat inside/cold outside..the glass windows got so fogged up that one couldn't really see much at all...so perhaps, winter isn't really the best time to do the tour. read more