The Tanninim in Genesis 1:21

Of particular interest is the specific attention paid to the "great creatures of the sea" in verse 21. Here the author returns to the verb he has not used since verse 1, bara', and which will only be used again in this chapter in verse 27. This use raises the significance of these creatures. In the ancient world the cosmic seas were populated with creatures that operated against the ordered system. Whether antithesis or enemy, they were viewed as threats to order, as they inhabited the region that was itself outside of the ordered system. This is the very reason why the author of Genesis would single them out for comment. Since there is no cosmic warfare or conquest in Genesis as is sometimes part of the ancient Near East picture, the text indicates that these creatures are simply part of the ordered system, not enemies that had to be defeated and kept in check. In Genesis these creatures are fully under God's control.

— John Walton, The Lost World of Genesis One, p.65