Modern cosmology is obviously very different from ancient cosmology, as the ancients were not scientists. We know the earth is a spherical-shaped planet rotating in place while revolving around the sun. But for ancient-minded people, their conception of the earth was very different. In their minds, the earth was flat with a crystal snow-globe-like ceiling called an “expanse” or “firmament” that was held up and supported by the earth’s pillar-like mountains. This crystal expanse held back water from falling out of the sky—except for when the “windows of heaven” would be opened in the expanse so that water would pour through as rain. (Note: This is everything you’re fighting for when you take a “literal view” of the creation story of Genesis instead of a “literary view.”) Above this expanse were the heavens, and at the highest part of the heavens was God himself.
In my opinion, the “crystal sea” of God’s throne room in the book of Revelation should be thought of as the expanse. John is borrowing from Ezekiel 1:22 which talks about the four living creatures of God’s throne room. Ezekiel mentions that above these creatures’ heads was “the likeness of an expanse, shining like awe-inspiring crystal.” Ezekiel 1:26 also mentions that this expanse is sapphire-like and that God’s throne sits on top of it. And that mention of sapphire leads us to Exodus 24:9-11 where Moses and 70 elders ate dinner up on a mountain, right under the sapphire expanse, through which they could see God’s feet.
So what is the crystal sea of Revelation? I think it’s John taking all of the conceptions of the expanse and combining it into a picture his audience understood. God’s throne is seated in the heaven of heavens, above the watery expanse/the sapphire firmament/the crystal sea—right where anyone would expect to find Him cosmologically located according to the Scriptures.