With two seas and a lake- and river-filled interior, don’t be surprised to find yourself in a boat at some point or other. For basic transport, boats are primarily used when travelling to or between the East Frisian Islands in Lower Saxony; the North Frisian Islands in Schleswig-Holstein; Helgoland, which also belongs to Schleswig-Holstein; and the islands of Poel, Rügen and Hiddensee in Mecklenburg-Western Pomerania. Scheduled boat services operate along sections of the Rhine, the Elbe and the Danube. There are also ferry services in areas with no or only a few bridges as well as on major lakes such as the Chiemsee and Lake Starnberg in Bavaria and Lake Constance in Baden-Württemberg.
From around April to October, local operators run scenic river or lake cruises lasting from one hour to a full day.
Read more: http://www.lonelyplanet.com/germany/transport/getting-around#ixzz2dQfAHBKq