There are a number of possible meanings of the sea,
all of which appeared already in the book of Revelation:
The literal bodies of water on this earth in distinction from dry land (Revelation 5:13; Revelation 7:1–3)
The place where goods are most easily transported, and so a metaphor for wealth (Revelation 18:19)
A representation of the floor beneath God’s throne (Revelation 4:6; Revelation 15:2)
The origin of evil forces (Revelation 13:1), and so representative of teeming, unstable, and threatening nations (Revelation 17:1; Revelation 20:13)
Perhaps we need not select between these possibilities. The point of Revelation 21:1 (as will become more evident from Revelation 21:3) is the (re)unification of heaven and earth, God’s dwelling and man’s. None of these various understandings of the sea
will threaten or hinder the harmony of God’s dwelling with people.
1 Then I saw a new heaven and a new earth, for the first heaven and the first earth had passed away, and the sea was no more.