Where are those blasted back copies of the “Old Farmer’s Almanac” when you need them? To the best of my memory, the Seven Seas are:
Black Sea (?)
Though it certainly is used in the meaning of having been all over the world, I think it refers specifically to the major waterways of the world where most of the international marine traffic is to be found.
Then again, the phrase may be way older than the discovery of the Pacific and besides it’s late and I’ve had too much Bass Ale. Forgive me
Cave Diem! Carpe Canem!