Adam Clarke on Revelation 21:1

A new heaven and a new earth - See the notes on 2Pet 3:13 : The ancient Jews believed that God would renew the heavens and the earth at the end of seven thousand years. The general supposition they founded on Isa 65:17.

There was no more sea - The sea no more appeared than did the first heaven and earth. All was made new; and probably the new sea occupied a different position and was differently distributed, from that of the old sea.

However, with respect to these subjects as they stand in this most figurative book, I must express myself in the words of Calmet: Vouloir dire quels seront ce nouveau ciel, et cette nouvelle terre, quels seront leurs ornamens et leur qualite, c'est a mon avis la plus grande de toutes les presomptions. En general, ces manieres de parler marquent de tres grands changemens dans l'univers. "To pretend to say what is meant by this new heaven and new earth, and what are their ornaments and qualities, is in my opinion the greatest of all presumptions. In general these figures of speech point out great alternations in the universe."