The answer to this question lies in the symbolic meaning of the sea in the Jewish pool of imagery. In the Old Testament, the sea is often portrayed as hostile to creation and to the people of God. In Psalm 104:5-9, the water is portrayed as fleeing from God at his rebuke. The mighty waters are portrayed as the enemy of God in Psalm 18:15-27. Of great importance for understanding sea imagery is Daniel 7 where the beasts which symbolize the foreign oppression of Israel come up out of the sea. Lastly, and perhaps most importantly, the image of the sea sometimes recalls God’s deliverance of Israel at the Red Sea and indicates the future hope of the people of God for a new exodus event, a future and ultimate rescue from oppression (Isaiah 51:10). In Jewish imagery, then, the sea can symbolize that which is opposed to God and to his people, the source of beastly and oppressive forces over the people of God, and the event of the exodus along with the hope that God will once again deliver his people.
Each of these themes is picked up in the development of sea imagery in Revelation. The sea first appears in Revelation 4 where there is something like a sea of glass before the throne of God (6). This is an image of God’s sovereignty over the sea and all that it symbolizes and is consistent with the Old Testament imagery of God rebuking or exercising his sovereignty over the sea. Further, like the beasts in Daniel 7, a beast arises from the sea in Revelation 13. This beast has characteristics from each of the beasts in Daniel 7 and appears to be an amalgamation of them. This confirms that in Revelation, as in Daniel, the sea is seen as the source of beastly oppression of the people of God. The sea of glass appears again in Revelation 15:2. This time, though, those who conquered the beast are standing upon the glassy sea and they are singing the song of Moses and the song of the lamb. Again, the imagery in the vision is similar to the imagery in the Old Testament. The song of Moses recalls the exodus and God’s deliverance of Israel at the Red Sea. It is clear that the image of the sea in Revelation corresponds to the standard Jewish symbolism associated with the sea.
We may suppose that the disappearance of the sea in Revelation 21:1 means that, in the new creation, all that which is opposed to God, his people, and his creation will be removed. All powers which oppress the people of God will be done away with. The people of God and creation with them will pass through the sea once again coming to freedom and life. This is confirmed by a few clues in chapter 21 itself. First, 21:1-5 has a chiastic structure which associates, “the sea is no more”(1c), with, “Death, sorrow, and crying are no more” (4a-c). With the disappearance of the sea, that which oppresses the people of God disappears as well. Second, Revelation, 21:4 echoes Isaiah 51:10-11 where the drying of the sea and the anticipation of a new exodus are associated with the doing away of sorrow and sighing.
The Old Testament background and the symbolism of the sea in Revelation both indicate that the disappearance of the sea in Revelation 21:1 means that God’s work of ultimate new creation will be a new event of salvation in which the people of God are fully and finally delivered from all forces of harm and oppression. Creation itself will fully and finally experience a new exodus from all opposition. The elimination of the sea symbolizes the final doing away with evil and the full redemption of the people of God.