It is a term known worldwide, used in many proverbs and great quotations. It is a figurative reference to all the bodies of water in the planet. It could be referencing the oceans that are : Arctic, Antartic, North, South Pacific, North and South Atlantic and The Indian Sea. But these oceans were not even known when this term was coined so, the creator and the man who made it mainstream to say. Rudyard Kipling, he used this phrase a lot in many of his 1870 poems. So he probably meant the following seas, Mediterranean, Red Sea, Indian Ocean, Persian Gulf, China Sea, West and East African Sea.