The reference is to the sea no longer existing (in all the above possibilities). The phrase John uses recalls Revelation 18:21–23, where various elements that characterized life in Babylon were “no more,” that is, no longer existed. The absence of the sea speaks to the fullness of the unity between heaven and earth; there no longer is anything else.
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.