The Seven Seas has been used to describe the world great water bodies for a long time.
In the ancient times, the Seven Seas are the Mediterranean Sea, the Adriatic Sea, The Black Sea, The Red Sea, the Arabian Sea, the Persian Gulf and the Caspian Sea.
Then before the Fall, it is the North Atlantic Ocean, the South Atlantic Ocean, the North Pacific Ocean, the South Pacific, the Arctic Ocean, the Southern and Indian Ocean
Then when the era of the Seven Great Powers began, the name of the Seven Seas also had changed.
There is the Pandemonium Sea which is basically the renaming of the South Pacific Ocean. There are also the other seas named after the Seven Great Power.
The World government called the sea under their control the Gaia Sea. Sometimes they would change the name
It is the same for the other world powers.