John Gill Commentary Jeremiah 51:42

"The sea is come up upon Babylon; she is covered with the multitude of the waves thereof." — Jeremiah 51:42 (ASV)

The sea is come up upon Babylon. A vast army, comparable to the great sea for the multitude thereof, even the army of the Medes and Persians under Cyrus; so the Targum, "a king with his armies, which are numerous like the waters of the sea, is come up against Babylon:"

she is covered with the multitude of the waves thereof; being surrounded, besieged, surprised, and seized upon by the multitude of soldiers in that army, which poured in upon it unawares.

Some think here is a beautiful antithesis, between the inundation of Cyrus's army and the draining of the river Euphrates, by which means he poured in his forces into Babylon.