The firmament is simply a space or an expanse. It’s what we call “outer space.” It contains the sun, moon and stars found in Gen. 1:14-19.
The waters below the firmament are the seas of the earth, (Gen. 1:9-10). The waters above the firmament are also called the sea (Job 41:31). They are beyond outer space and enclose the entire universe. From Ps. 148:4, we see that there are “waters that be above the heavens.” And since these waters completely enclose the universe, they are also “below” the earth. Deut. 5:8 speaks of “the waters beneath the earth,” (see Ps. 136:6).
As a result of this firmament, there are three heavens. The first heaven is from the earth to the clouds; the birds fly in this “open firmament of heaven,” (Gen. 1:20). The second heaven is from the clouds to the waters above the firmament, (Gen. 1:8). The third heaven is above the waters where Paul “was caught up into paradise,” (2 Cor. 12:1-4).
In the earth, sinners without Jesus Christ are under the wrath of God (Jn. 3:36). Indeed, before we were saved, we were separated from God in heaven by that great body of water above the firmament. So, the wrath of God is like the waves of the ocean beating on a man (Ps. 88:7). Consequently, when God saved us, he sent “fishers of men,” (Matt. 4:19) to draw us out of the water (Ps. 18:16).
In the new heaven and the new earth (Rev. 21:1-3), “God will dwell with [men] … and God himself shall be with them, and be their God.” Therefore, the universe will no longer be enclosed in water to separate men from God and so there will be “no more sea” above the universe, (Rev. 21:1).