As a prophetic symbol, the sea is mentioned in both Daniel and Revelation. It is defined in Genesis 1:10 as "the gathering together of the waters." Water, as a prophetic symbol, is defined in Revelation 17:15 as "peoples, and multitudes, and nations, and tongues." So a prophetic sea is a geographical area containing many peoples and multitudes and nations and tongues. The specific reference is to what we call the Old World: Europe and the Near East.
Examples: