Water on the whole planet is one whole. If it rains in Bulgaria today, after a while this water may reach America. The water circulation is so universal that it includes both fresh water basins and the cloud cover, and the seas and the oceans.
Unfortunately the whole process is too susceptible to human activity. Water pollution at one point surely affects all other points.
There is no better way to illustrate the connection of water than a voyage around the world. The normal voyage involves sea routes but in this case we practically prove the connection between the river networks on the whole planet.