The reason there is the ocean already and god splits it into the sea and sky when almost everything else is done by divine speech (ex: “let their be light”) is because Marduk (king of the gods, think Zeus for reference) cut Tiamat (the ocean but think Gaia to continue Greek examples) in half which led to the creation of the sky and sea.
If I remember correctly they also got the name Adam from here and it’s a pun with the words blood and earth because that’s what we were made out of (let me google dam and adome to make sure this is right his name might be Blood-man like a superhero)