Jeremiah 51:41

"How is Sheshak taken and the praise of the whole earth seized! how is Babylon become a desolation among the nations! The sea is come up upon Babylon; she is covered with the multitude of the waves thereof. Her cities are become a desolation, a dry land, and a desert, a land wherein no man dwelleth, neither doth any son of man pass thereby. And I will execute judgment upon Bel in Babylon, and I will bring forth out of his mouth that which he hath swallowed up; and the nations shall not flow any more unto him: yea the wall of Babylon shall fall."

"How is Sheshak taken ..." (Jeremiah 51:41). This is an ashbash for Babylon. See under Jeremiah 51:1, above, and under Jeremiah 25:26.

"The sea is come up upon Babylon ..." (Jeremiah 51:42). This is a metaphor for the destroying army, composed of many nations under the lordship of Cyrus.

"I will bring forth out of his mouth ..." (Jeremiah 51:44). This reveals the true identity of the one who swallowed up Jerusalem. It was not a sea-monster at all, but Babylon, because Babylon was the one that God forced to disgorge himself of that which he had swallowed.

Jeremiah 51:41-43 here are the same as Jeremiah 6:22-24. See my comments there.