In ancient near eastern cosmology the idea of a primordial ocean out of which things came was common. We can see it, for example, in the Enuma Elish, the Babylonian creation myth where the primordial ocean is personified as the goddess Tiamut; Tiamut gave rise to the gods, then the gods battled it out for supremacy--from their violent warring, their spilled blood etc, came land, mountains, trees, animals, and human beings.
In Genesis, on the contrary, God precedes the primordial ocean and He deliberately gives shape and form to the universe; He takes the unformed earth and gives it form--He separates light and dark, water and sky, land and sea, and then He populates these spaces with creatures to rule them. He gives the sun, moon, and stars to rule day and night, He gives birds and fish to rule sky and sea, and He gives beasts and creeping things to rule the land--then He makes human beings to be His representative in creation, as the image-bearing creature.
The author isn't interested in where the primordial matter of the cosmos came from, the author is interested in describing the God who ordered creation and ultimately man's place in God's order.
-CryptoLutheran