I cannot read God's mind, of course. I am also aware that interpreting Revelation is a tricky business. But it sounds as if John meant exactly what he said, and perhaps he did. Possibly there will be no sea, no waves, no tides, no whales, no plankton, no kelp, no sea horses, no sponges in (or around?) the new habitation of mankind with the heavenly beings.
This has always (dare I say it?) disappointed me. I like the ocean, and ocean life. Some have suggested that John wrote this because he was imprisoned on an island, with no escape, surrounded by the sea. But would God allow his Word to be so influenced by the dislike of one man? I doubt it. I just don't know why that verse is in Revelation.
The Old Testament has a couple of passages that seem to modify the picture of a new, sea-less cosmos. One of these is Genesis 1. In verse 10, Genesis says: God called the dry land Earth, and the waters that were gathered together he called Seas. And God saw that it was good. (ESV)
In verses 20-23, it says: 20 And God said, “Let the waters swarm with swarms of living creatures, and let birds fly above the earth across the expanse of the heavens.” 21 So God created the great sea creatures and every living creature that moves, with which the waters swarm, according to their kinds, and every winged bird according to its kind. And God saw that it was good. 22 And God blessed them, saying, “Be fruitful and multiply and fill the waters in the seas, and let birds multiply on the earth.” 23 And there was evening and there was morning, the fifth day. (ESV) The description of the fifth day (whatever a day is, in this context) concludes, also, with the phrase about God seeing what he had created as good. So the sea, and the creatures in it, were originally declared to be good.
I recently found another passage that seems to relate, namely Ezekiel 47:6-12:
So, in this passage, apparently also describing the way things will be after Christ's second coming -- and, also, as prophecy, tricky to interpret -- there will be a sea, with water-dwelling creatures in it.
The Blueletter Bible has commentaries. Two of them bear on this verse. David Guzik says that, to the Hebrews, the sea represented evil, or God's enemies, and cites Psalm 89:9 and Isaiah 57:20 as proof of this. He also says that the sea has already appeared in Revelation, in 13:1, where the beast comes from the sea, and 20:13, as a place holding the dead. A. R. Fausset says this: The sea is the type of perpetual unrest. Hence our Lord rebukes it as an unruly hostile troubler of His people. . . . As the physical corresponds to the spiritual and moral world, so the absence of sea, after the metamorphosis of the earth by fire, answers to the unruffled state of solid peace which shall then prevail.
If I understand them correctly (and they understand Revelation correctly) John was not speaking literally. Based on the probable symbolic use of the sea by John, on its original goodness, and Ezekiel's statements about the Dead Sea, perhaps there will be some sort of sea in the new heavens and new earth. We'll see, I hope.
Thanks for reading.
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See this post for more on this topic.
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Added February 2, 2015:
In this post, Bible scholar John Walton says, about the Final Kingdom, ". . . there’ll be no sea, which is the place of non-order in the ancient world."