These vast oceans are the seven seas that we now list—however, the concept of the seven seas is ancient and also very variable. We know the Sumerians had a list (from a reference in the hymn of the Enheduanna) but not what was on it. By the time of the Phoenicians, there was a canonical list for the seven seas within the Mediterranean, upon which their black ships traded. Working west from their homeland, there was the Aegean, Ionian, and Adriatic, whilst west of Sicily stretched the Tyrrhenian, Ligurian, Balearic, and sea of Alboran (the straits of Gibraltar).
For a Muslim Arab trader the seven seas referred to that vital sinew of trade that took them east to the coast of China, beginning with the Persian Gulf, then the Gulf of Khambhat (Sind and Gujarat), Harkand (the Indian Ocean and the Bay of Bengal), Kalah (the Malacca straits), Salahit (the straits of Singapore), Kardani (the waters of Siam) and Sanji (the South China sea). Medieval Christian traders, such as the Venetians and Genoese, made lists of seven that included the Adriatic, Black Sea, Caspian Sea, Red Sea, Mediterranean, Arabian Gulf and Indian Ocean.”
Excerpted from: Rogerson, Barnaby. Rogerson’s Book of Numbers: The Culture of Numbers–from 1,001 Nights to the Seven Wonders of the World. New York: Picador, 2013.