To the ancients, "seven" often meant "many," and before the fifteenth
century, the many seas of the world were:

*       the Red Sea
*       the Mediterranean Sea
*       the Persian Gulf
*       the Black Sea
*       the Adriatic Sea
*       the Caspian Sea
*       the Indian Ocean


see: http://www.whoi.edu/info/seven-seas.html

All the best


Riccardo Giannuzzi-Savelli