Avast thar, matey, as we raise the anchor and set sail for the seven seas!
"What seas be these," ye land-lubbers say?
Well, honestly, any seas, just as long as there's a lot of 'em.
While pirates and ancient mariners may have bragged about sailing the seven seas, the phrase is merely figurative. The dictionary explains it as meaning "all the oceans of the world."
In ancient and medieval Europe and Arabia, the seven seas were some combination of these nine bodies of water: the Adriatic Sea, Aegean Sea, Arabian Sea, Black Sea, Caspian Sea, Indian Ocean, Mediterranean Sea, Persian Gulf, and Red Sea. Notice how they're all clustered around the Mediterranean -- these so-called sailors had yet to venture out of their backyards.
During the Age of Discovery, from 1450 to 1650, this informal list was sometimes rearranged to be these seven navigable seas: the Arctic Ocean, Atlantic Ocean, Caribbean Sea, Gulf of Mexico, Indian Ocean, Mediterranean Sea, and Pacific Ocean. Bigger, but still not all-encompassing.
Only in more recent times did the list go worldwide as the Arctic Ocean, Antarctic (or Southern) Ocean, Indian Ocean, North Atlantic Ocean, South Atlantic Ocean, North Pacific Ocean, and South Pacific Ocean. Of course, that's something of a stretch, because it cuts two oceans in half. And the world's oceans are actually connected, so they're all one, great, big sea.