The Revelation speaks of there being no more sea because of what "the sea" represents symbolically. Not that the oceans and seas and large lakes suddenly vanish from the face of the planet. "The sea" represented danger in the ancient world, storms came from the sea, invading peoples came from the sea. The Philistines, for example, were very likely related to the "Sea Peoples" mentioned as being at least partly responsible for the Bronze Age Collapse that saw the end of nearly every major civilization and power in the eastern Mediterranean. The Egyptians speak of them and how Egypt nearly ended, just like the Hittites and Mycenaeans did.
The sea represented danger, powerful enemies lay across the sea, the storms were a danger to anyone who would risk going on board a ship into the sea.
The future world will be a world without the threat of danger, death, and violence--but a perfect world in which God's peace and justice abound.
-CryptoLutheran