River cruises are available from the city of Passau, which is located 120 mi/194 km northeast of Munich. Passau can be reached via car or bus on the A92 autobahn, or via train (about a two-hour ride). http://tourism.passau.de/PassausHighlights/DanubeRiverboatTrips.aspx.
The cruises vary in destination and duration, but most travel on the Danube river. A sample cruise is from Passau to Budapest, Hungary, then back to Passau, stopping in Linz and Vienna, Austria, as well as Bratislava, Slovakia,...