A continental divide is a line of raised ground that creates a border between two watersheds. Water falling on one side of the line travels to one ocean, and water on the other side travels to another, perhaps on the other side of the continent.
Because the exact border between bodies of water is usually not very easy to see, the continental divide is not always clear. The International Hydrographic Organization's publication Limits of Oceans and Seas describes the exact boundaries of oceans, but it is not recognized around the world. A complication is that some rivers empty into deserts or inland seas, and so do not end up in the ocean.