I think people miss the obvious when trying to pinpoint the meaning of the passage:

 

Revelation 21:1

¹Then I saw a new heaven and a new earth; for the first heaven and the first earth passed away, and there is no longer any sea.

 

There are two kinds of “sea” mentioned in the Revelation, there are the references to those waters that constitute the “seas” which are upon the earth and then there are references to the “sea” in the heavenly tabernacle as follows:

 

Revelation 4:6

⁶and before the throne there was something like a sea of glass, like crystal; and in the center and around the throne, four living creatures full of eyes in front and behind.

 

Revelation 15:2

²And I saw something like a sea of glass mixed with fire, and those who had been victorious over the beast and his image and the number of his name, standing on the sea of glass, holding harps of God.

 

Revelation 20:13

¹³And the sea gave up the dead which were in it, and death and Hades gave up the dead which were in them; and they were judged, every one of them according to their deeds.

 

 

You will note that the sea in Revelation 20:13 is a repository for the souls of the dead, and is listed along with “death” and “ Hades” as places which give up their dead at the final judgment.  I would suggest that the “sea” here is not an earthly body of water, but rather is that “sea” which John saw in the heavenly tabernacle. This “sea” served as the pattern of the “sea” which was created for the tabernacle and later the temple and its purpose was for the cleansing of the priests. I would also suggest that this heavenly “sea” is where the souls of the righteous reside while they await their resurrection. Yes there will be those who are considered righteous who will die during the millennial kingdom. They are not immediately resurrected upon their death, but are raised with those souls who reside in death and Hades at the final judgment. I believe that paradise resides within this heavenly “sea”.

 

Revelation 20:11–15

¹¹Then I saw a great white throne and Him who sat upon it, from whose presence earth and heaven fled away, and no place was found for them. ¹²And I saw the dead, the great and the small, standing before the throne, and books were opened; and another book was opened, which is the book of life; and the dead were judged from the things which were written in the books, according to their deeds. ¹³And the sea gave up the dead which were in it, and death and Hades gave up the dead which were in them; and they were judged, every one of them according to their deeds. ¹⁴Then death and Hades were thrown into the lake of fire. This is the second death, the lake of fire. ¹⁵And if anyone’s name was not found written in the book of life, he was thrown into the lake of fire.

 

You will notice in this passage that death and Hades (the repository for the souls of the wicked) are thrown into the lake of fire,  and the very next passage we see is the mention that there is no longer any sea when the new heavens and earth are created. This sea is no longer needed.

 

Revelation 21:1

¹Then I saw a new heaven and a new earth; for the first heaven and the first earth passed away, and there is no longer any sea.

 

This says there is no longer any sea. The reason being is because there will no longer be anyone who dies, all the righteous will be raised to inherit their glorified eternal body. There is no more need for a repository for righteous souls.

 

I believe that the sea which is no more, is not an earthly body of water, but the heavenly sea which John describes throughout the revelation. Seems pretty obvious to me.