As gases cool, they become fluids, which in mythical parlance are called 'waters', hence the writers of the Genesis myth also used this analogy. In this case it was likely that the 'waters' in question were the fluid state of the rocks and the actual water evaporating and being created by the fusion of Hydrogen and Oxygen. The firmament becomes like a hole keeping the two expanses of water apart, but he then names the upper 'ocean' heaven and leaves the other areas nameless. Heaven is therefore seen as a place full with the force of life.
