The phrase "sail the Seven Seas" has had different meanings to different people at different times in history. The term "Seven Seas" is mentioned by ancient Hindus, Chinese, Persians, Romans and other cultures. The term historically referred to bodies of water along trade routes and regional waters; although in some cases the seas are mythical and not actual bodies of water.By Kim Ann Zimmermann, Live Science
The term "Seven Seas" has evolved to become a figurative term to describe a sailor who has navigated all the seas and oceans of the world, and not literally seven.
The Oxford Dictionary
The seven seas
All the oceans of the world (conventionally listed as the Arctic, Antarctic, North Pacific, South Pacific, North Atlantic, South Atlantic, and Indian Oceans).