What I would do by boat, is visiting the 'Pfaueninsel', something similar to Governor's Island in NY. (Well, not really, but ...)
You could than go by boat to the last Hohenzollern royal residence, the Cecilienhof Palace (built in 1913-1917 in an English manor house style for Crown Prince William and his wife Cecilie).
This palace became famous in history as the 1945 Potsdam Conference venue - and is really not one of the more "Berlin secrets" to visit but it's usually not overrun - and does have some very, very interesting showrooms (Quite interesting, how little the Americans knew about Germany when they divided it.)
Just around the corner, in an easy walking distance, there aren't only some pretty old villas but the former KGB prison as well.