A ferry trip courtesy of public transport
Every day, the F10 ferry crosses Wannsee lake to Kladow, a 20-minute trip that costs nothing more than a standard A-B ticket (it's part of Berlin's official transport network).
En route, you'll pass the islands of Schwanenwerder and Pfaueninsel, cruise past sandy Wannsee beach (below left), and enjoy far-reaching views of the Havel River.
It's one of Berlin's best sightseeing bargains, and even if the functional ferry can't quite be compared to a pleasure boat (there are no outside decks and you'll need to take your own snacks and drinks), it still provides a near-perfect budget excursion on the water.
Incidentally, Berlin also has 5 other public ferry lines, most serving the Treptow-Köpenick area. They're not as easy to reach as the F10 (and crossings are generally just a few minutes long), but the F23 plies an idyllic, 25-minute route from the Eastern end of Muggelsee lake to Kruggasse.
And for real curiosity-lovers, the F24 is Germany's only rowing boat ferry - a man-powered, 60-second jaunt from one side of a river inlet to another.
Once you've reached Kladow, visit the historic waterside Kulturpark Cafe (once home to Bismarck's mother), or venture further afield to the Groß-Gleinicker See, one of Berlin's cleanest, clearest lakes.
F10 Ferry from Wannsee to Kladow - the Wannsee landing is a just a short, signposted walk from Wannsee S-Bahn station. An A-B ticket from central Berlin can be used for both the train / bus journey to Wannsee, and the ferry trip itself.
Summer timetable: hourly service Mon-Fri from 6am-7pm; Sat 7am-7pm; Sun and public holidays 10am-6pm.
Winter timetable: Mon-Fri 6am-6pm; Sat &am-7pm; Sun and public holidays 10am-4pm.