Why is there no more sea in the new earth?
In the new earth there is no more sea because God wants no separation or division between nations and no division among His people; the sea that once divided the nations of the earth is removed forever.
When Revelation describes the new heaven and new earth, it specifically says there was no more sea. This is not a small detail. The sea in the old creation stood as a barrier that separated nations from one another, keeping people apart by wide oceans and divided lands. God removes that barrier because He desires no more separation, no division of nations, and no divisions between the people of God.
In the new earth, the tabernacle of God is with men, and He dwells with them as one people. There is no more death, sorrow, crying, or pain, because the former things have passed away. Taking away the sea fits with this whole picture of unity: God is bringing all things together under Himself, and the very geography of the new earth reflects that oneness rather than division.
This is also connected to mankind being restored to one language, so that all may call upon the name of the Lord and serve Him with one accord. Just as language once divided people, the sea once divided nations by distance and geography. In the new creation, both barriers are gone. The earth becomes a paradise again, made even better than before, with nothing left standing between the nations and nothing left standing between the people of God.
The scriptures
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.
And I heard a great voice out of heaven saying, Behold, the tabernacle of God is with men, and he will dwell with them, and they shall be his people, and God himself shall be with them, and be their God. And God shall wipe away all tears from their eyes; and there shall be no more death, neither sorrow, nor crying, neither shall there be any more pain: for the former things are passed away.
For then will I turn to the people a pure language, that they may all call upon the name of the LORD, to serve him with one consent.
From the books
This answer is drawn from these passages in the Mary Banks Faith Library.