On day 2 of creation, God created a physical structure separating waters above and below it:
Genesis 1:6-8
[6] Then God said, Let there be a firmament in the midst of the waters, and let it divide the waters from the waters.
[7] Thus God made the firmament, and divided the waters which were under the firmament from the waters which were above the firmament; and it was so.
[8] And God called the firmament Heaven. So the evening and the morning were the second day.
The Book of Jubilees provides the detail of the waters being created on day 1:
Jubilees 2:2
[2] For on the first day He created the heavens which are above and the earth and the waters and all the spirits which serve before him – the angels of the presence, and the angels of sanctification…
Ezra repeats these understandings:
2 Esdras 6:41
[41] Again, on the second day, you created the spirit of the firmament, and commanded it to divide and separate the waters, so that one part might move upwards and the other part remain beneath.
2 Esdras 16:58-59
[58] He has confined the sea in the midst of the waters; and by his word he has suspended the earth over the water
[59] He has spread out the heaven like a dome and made it secure upon the waters;
Levi was shown these waters in his vision of the heavens:
Testament of Levi 2:7
[7] And I entered from the first heaven, and I saw there a great sea hanging.
Peter addresses the willful ignorance of mockers in the last days regarding this reality:
2 Peter 3:5
[5] They willfully ignore this: Long ago the heavens and the earth were brought about from water and through water by the word of God.