John Calvin 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 Prophet here employs a comparison to more fully confirm his prophecy concerning the destruction of Babylon. For, since it was incredible that it could be subdued by the power or forces of men, he compares the calamity by which God would overwhelm it to a deluge.

He then says that the army of the Persians and Medes would be like the sea, for it would irresistibly overflow. Just as the sea swells when a storm rises, so, he says, the Medes and Persians would come with such force that Babylon would be overwhelmed by a deluge rather than by the forces of men. So now we understand the Prophet’s meaning when he says that Babylon would be covered with waves when the Medes and Persians came.