What are the Seven Seas? 

It's an ancient term describing what were believed to be all the
seas and oceans of the world. Many people believe the seven are
what we now know as the Arctic, Antarctic, North Pacific, South
Pacific, North Atlantic, South Atlantic and Indian oceans.
However, the phrase has no literal meaning and is used to
romanticize sailing. It came into use before some of the oceans
were even known to exist.