Appears in the Enuma Elish as the first god with his wife Tiamat, existing before the heavens and earth had come into existence. Apsu seems to be salt water, while Tiamat is fresh water. They mixed their waters and gave birth to the gods.
(Enuma Elish @ https://www.worldhistory.org/article/225/enuma-elish---the-babylonian-epic-of-creation---fu/)