A prophetic "sea" is a large geographical area containing many different nations, and its use in prophecy specifically refers to the heavily populated portion of the eastern hemisphere.
Genesis 1:10 says, "And God called the dry land Earth." In contrast to the "sea," the "earth" symbolizes the part of the world which, at the time to which the prophecy applied, was not filled with nations. Specifically this symbol is applied to the land of the United States during the period of its early settlement.
Examples: