1 Here καί (kai) has been translated as “then” to indicate the implied sequence within the narrative.
2 For the translation of ἀπέρχομαι (aperchomai; here ἀπῆλθαν [apēlthan]) L&N 13.93 has “to go out of existence—‘to cease to exist, to pass away, to cease.’ ”

In other versions

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.

Then I saw “a new heaven and a new earth,” for the first heaven and the first earth had passed away, and there was no longer any sea.

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.

Then I saw a new heaven and a new earth, for the old heaven and the old earth had disappeared. And the sea was also gone.

And I saw a new heaven and a new earth, for the first heaven and the first earth had passed away, and the sea did not exist any longer.

And I saw a new heaven and a new earth: for the first heaven and the first earth were passed away; and there was no more sea.

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