Mythologically, the sea has always represented chaos and disorder. Often, a great sea monster is connected with it in creation myths. But one doesn't have to search far and wide in the myths of other cultures in order to find that significance of the sea as chaos. The Bible makes ample use of it, including the Book of Revelation itself. The Bible's own creation stories most often make use of the sea as a sign of chaos.
Genesis begins with a picture of sea alone.
In the beginning, when God created the heavens and the earth, all that existed was the chaos of the sea. Into this picture, God's breath moved over the waters and began to bring order out of the disorder. The earth marked out a safe place from the waters below, and the firmanent held back the waters above. In contrast to the beginning, when there was nothing but the sea, Revelation's picture of the new heaven and the new earth is one where there is no sea. All the chaos and disorder has given way to God's loving power of creation.