It might seem odd that nations with no access to the ocean would maintain naval forces, but many do. Kazakhstan and Azerbaijan, for example, keep many naval vessels on the enormous Caspian Sea. Switzerland and Burundi have armed patrol boats on their border lakes. These forces, however, are not separate military entities, but integrated into the other armed forces. What makes the following nations unique is that they maintain separate military organizations identified as navies, but these forces have access only to inland rivers and lakes.
Boliva once had a substantial coastline, but lost it after a defeat by Chile in the War of the Pacific (1879-1884). Bolivia has never forgotten this blow and did not disband its navy, though it lacked anywhere on the sea to base it. Instead, it rebuilt its flotilla of ships on Lake Titicaca, a large lake that it shares with its wartime ally Peru. Every year, on March 23, the nation commemorates "Bolivia Sea Day" and its representatives at the United Nations call upon Chile to return the territory. The Chilean government is not adverse to the notion of retuning a narrow corridor of territory along its northern border, but Peruvian objections and other issues have so far prevented a resolution of this border dispute. In the meantime, the Bolivian Naval Academy trains navy and marine corps officers to lead the 4,500-man force. The sailors experience actual sea duty serving in the navies of friendly Latin Amercian neighbors. And the fleet of fourteen patrol boats, six transports, two hospital ships, and two research vessels patrols Lake Titicaca and several tributary rivers of the Amazon -- all in preparation for the day when Bolivian territory again reaches to the Pacific Ocean.
Paraguay, to the east, never had access to the sea. But its economy is tied to the Rio Paraná and the Rio Paraguay, so it has longed maintained a substantial brown water navy. This would prove essential to preserving its independence during the War of The Triple Alliance (1864-1870) against Argentina, Brazil, and Uruguay. It fought gallantly but unsuccessfully at the Battle of Riachuelo (1865), one of the largest riverine engagements in modern naval history. Today, its 2,500 sailors, including 500 marines, protect the nation's rivers on sixteen patrol boats, three amphibious assault ships, and seven support vessels. After basic training in-country, Paraguyan sailors gain experience in the navy of Argentina before assuming their duties at home.
Yugoslavia once had eighty warships in its fleet. But after the dissolution of that state in the 1990s, Serbia's access to the sea was limited to its federation with tiny Montenegro, where it based a 8,000-man navy with an assortment of frigates, missile boats, minesweepers, and landing craft. When that nation seceded in 2006, Serbia became landlocked. The remains of its seagoing navy were auctioned off by Montenegro, mostly to Egypt. Its forces on the Danube and other rivers (such as the old Yugoslav riverine assault ship pictured above) were subordinated to the army.
In the 13th Century, the Mongolian Navy was the largest in the world. It lost that fleet during two failed invasions of Japan. Since that time, it shrank down to nothing. In the 1930s, the Mongolian Navy was reborn under the auspices of the Soviet Union. It received one boat, the Sukhbaatar, to patrol Lake Hovsgal. The Soviet-made vessel was dragged overland to that lake. The ship was named after Damdiny Sukhbaatar, the Mongolian leader who drove China out in the 1920s. It, and its successor, the Sukhbaatar II, eventually sank.
But as of 2001, one vessel remains -- the tugboat Sukhbaatar III. It's manned by seven men, only one of whom knows how to swim. The ship is homeported at Khatgal, and it continues its regular patrols of Lake Hovsgal. But due to financial constraints, the Mongolian government privatized its navy in 1997. So now the Sukhbaatar III supplements its income by hauling freight across the lake. You can watch a documentary about the Mongolian Navy here.