Why would Jeremiah talk about drinking water when he's discussing foreign policy? It turns out these mighty rivers were more than just sources of water.
Jeremiah uses the great rivers of Egypt (the Nile, called Sihor) and Assyria (the Euphrates) as powerful metaphors.
The Water of Nations
These rivers represented the dominant empires of the time. To 'drink the waters' of Egypt or Assyria meant seeking alliances, drawing strength, and adopting the principles and even the idolatrous practices of these foreign powers.
A False Hope
God contrasts these 'rivers' with Himself, the 'fountain of living waters.' Just as the people were trying to find life-sustaining water from these foreign sources, they were actually turning away from the true source of life and help – God Himself. These alliances were like broken cisterns, unable to hold water and ultimately useless.