What are "the Seven Seas?"
The phrase "seven seas" is an ancient term used to describe all the seas and oceans of the world. The phrase has no definite meaning, which makes sense when you consider that it was coined before some of the world's oceans were known to exist. (It's also worth noting that "seven" was often used to mean "several" in times gone by.)
Brewer's Dictionary of Phrase and Fable claims it refers to the Arctic, Antarctic, North and South Pacific, North and South Atlantic, and Indian Oceans. One of several earlier interpretations has it referring to the Red Sea, Mediterranean Sea, Persian Gulf, Black Sea, Adriatic Sea, Caspian Sea and Indian Ocean. The Aegean Sea and Arabian Sea have also made their way onto similar lists.
—The Fact Monster