What are the “waters” of Gen 1:9, and why are some “under” and others “above” the firmament?

Since rains falls from the clouds of water vapor, and since there is usually some visible space between the ocean and such clouds (if there is enough ambient light), it might well make sense to say, “Let there be a firmament in the midst of the waters, and let it divide the waters (of the ocean) from the waters (of the clouds).” (Gen 1:6)