In the scheme of all features that make up the universe, the earth as a feature is too insignificant, and if that is the case where would the water/s come in? So if the sky was indeed the expanse that seperates the waters, since the expanse is in place for that purpose, the waters above that expanse would never be seen by man as cosmology claims that the expanse grows by day.
Let scripture answer scripture;
The deep in scriptures is the source of the waters, and it isn't physical but spirit, which means that the 'waters' that it's a source of are also 'spirit'.
Or, Who shall descend into the deep? (that is, to bring up Christ again from the dead)
Did Christ drown at sea?
Concerning the fall of Satan;
Thus saith the Lord GOD; In the day when he went down to the grave I caused a mourning: I covered the deep for him, and I restrained the floods thereof, and the great waters were stayed: and I caused Lebanon to mourn for him, and all the trees of the field fainted for him.
Satan is spirit. Likewise, his place of incarceration must also be spirit. The deep is always depicted as a source of 'spiritual material' for God's use(eg Psalm 139:1), and the abode of all spirit prisoners.
About the devils that messiah cast out of a lunatic;
And they besought him that he would not command them to go out into the deep.
They knew it for a 'nasty' place, so they asked for any banishment but the deep.
Their fear was that 'unpleasantness', the darkness that came up on face of the deep, that darkness that characterised the deep in many places it is cited. This is what was even on the face of the waters that the spirit of Elohim 'moved' upon.
The dwellers in the deep are often denoted as the 'deep'
For thus saith the Lord GOD; When I shall make thee a desolate city, like the cities that are not inhabited; when I shall bring up the deep upon thee, and great waters shall cover thee;
Bring up the deep and great waters?
He means judgments on the wayward people by allowing spirits from the underworld to come up and wreck havoc among them by ways that cause damnation.
This is the deep in Gen 1:6 whose waters are 'seperated' upon changing by the 'moving' of the spirit of Elohim upon the face of them.
That these 'waters' had to be seperated indicates distinct qualities in them that called for that seperation. So in a sense they were mixed. Later, they would be seperated from each other by the mysterious 'expanse.' The expanse was to be in the 'midst' of the waters. 'Midst' here doesn't mean 'middle' as many versions render the word. It alludes to circumstances, while 'middle' refers to a position.
This is the 'seperation' meant in Gen 1:6;
You are therefore to make a distinction between the clean animal and the unclean, and between the unclean bird and the clean; and you shall not make yourselves detestable by animal or by bird or by anything that creeps on the ground, which I have separated for you as unclean.
13 You shall have the Levites stand before Aaron and before his sons so as to present them as a wave offering to the LORD.
14 Thus you shall separate the Levites from among the sons of Israel, and the Levites shall be Mine.
Ezekiel 39:14 etc.
And they shall sever out men of continual employment, passing through the land to bury with the passengers those that remain upon the face of the earth, to cleanse it: after the end of seven months shall they search.
The spirit of Elohim that had moved upon the waters had imparted to some of them natures that were distinct from each other; to create a being or impart a nature to a creature is the work of the spirit of Elohim. Ref Psalm 104:30, Genesis 1:2.
This is why there was a need to 'seperate' the waters.
The expanse is that which seperates the entities into different realms by means of their natures, it was a physical barrier but a 'tweek in the natures.'
So the term 'ABOVE' in Gen 1:6 alludes to a higher essence as messaih refers in the two natures, one from below and the other from above in:
John 8:23 And he said unto them, Ye are from beneath; I am from above: ye are of this world; I am not of this world.
John 17:16 etc.
They are not of the world, even as I am not of the world.