Jamieson, Fausset, Brown comments on Rev 21:1
“No more sea — The sea is the type of perpetual unrest. Hence our Lord rebukes it as an unruly hostile troubler of His people. It symbolized the political tumults out of which “the beast” arose in Revelation 13:1. As the physical corresponds to the spiritual and moral world, so the absence of sea, after the metamorphosis of the earth by fire, answers to the unruffled state of solid peace which shall then prevail. The sea, though severing lands from one another, is now, by God’s eliciting of good from evil, made the medium of communication between countries through navigation. . . . The sea was once the element of the world’s destruction [global flood of Noah’s day], and is still the source of death to thousands, whence after the millennium, at the general judgment, it is specially said, “The sea gave up the dead . . . in it.” Then it shall cease to destroy, or disturb, being removed altogether on account of its past destructions.” (End quote)
Ed Steven’s comments:
This is certainly cosmological terminology relating to the nature of the New Creation and how it was qualitatively different (new in kind, not just new in time) from the old cosmos. The sea had such negative connotations throughout the Old Testament and in ancient culture generally. It was one of the places where the dead were buried. Many people left the land on boats never to be seen again. Their families were rightly concerned for their safety. The sea was considered to be a widow-maker, greatly feared by women and children. Evil Gentile armies from nations across the sea arrived on the shore of Israel to invade and conquer the land of Israel.
Thus it was considered to be the source of evil destructive things. The sea was a dangerous place to be. It was considered to be one of the gateways to the Underworld (Sheol-Hades). Some have suggested that the sea represents the Gentile nations and non-Israelites because John says that the great harlot “sits on many waters” (Rev 17:1) which are “peoples and multitudes and nations and tongues” (Rev 17:15). Bible references can be drawn to support this position: (Read) Isa. 60:5; Eph. 2:11-22; Col. 3:11; Gal. 3:28; 1 Cor. 15:28.
That could be the sea that is under consideration in Rev. 21:1, but it does not have to be. Augustine suggested that John was comparing the restless, chaotic, stormy, and unsettled nature of the ocean to the restless chaotic nature of the Gentile nations in the pre-Christian world. In the New World of Christianity, the Gentiles would no longer be a threat to God’s Kingdom which is spiritual in nature.
The Jews considered all the land on the whole earth to be centered and founded upon the huge rock (Mt. Moriah) upon which the temple was built. They believed that the land of Israel was at the center of the earth, with Jerusalem being in the center of the land, and the temple being in the center of Jerusalem, with its Holy of Holies at the very center of the city. The temple itself was divided into three regions (heaven, earth, and sea), moving outward from the Holy of Holies.
These three regions represented the three divisions of the cosmos (heaven above, earth beneath, and the sea). The new temple would not have the sea, implying that there would be no place that was dangerous and foreboding (no Gentile enemies). God’s presence would extend out of the Holy of Holies (heaven) to fill the earth and eliminate the negative threats, fears, and dangers of the sea. No longer would the Gentile nations beyond the seas be excluded from the Presence of God and His Kingdom. The believing Gentiles were now reconciled to God and made one with the believing Israelites in the Kingdom of Christ (The New Heavens and Earth).
[For a more in-depth study see eschatology “Study Series 16 Lesson 9 Rev. Chapter 21”]