Most of the world's dry land would have to be covered by rising seas before Mongolia could come close to having an ocean port. (The country's lowest altitude is 532 meters above sea level.) But Mongolia's position astride some of the world's highest mountains is no barrier to registering ships.
Since 2004, the Mongolian flag has been numbered among the world's flags of convenience for ship owners. There's even a photo of a ship with Mongolian registry at Coming Anarchy.