Socialist Federal Republic of Yugoslavia
Regarding rivers, the Danube was navigable throughout its entire course in Yugoslavia, linking the ports of Belgrade, Novi Sad, and Vukovar with Central Europe and the Black sea. Long stretches of rivers Sava, Drava and Tisza were also navigable.
Danube Flotilla (Soviet Union)
The Danube Flotilla was a naval force of the Soviet Navy's Black Sea Fleet during World War II (in Russia, called the Great Patriotic War) and afterwards, existing 1940–1941 and 1944–1960. The Flotilla operated on the Danube River and also, at times, on other rivers connected to the Black Sea.