After extensive literary review and online research including reader friendly sites such as the Library of Congress and Geography@About.com has shown that the idea of seven seas seems universal. Sumerians praised their gods for the seven seas as far back as 2300 BC. The Romans called the salt marshes around present-day Venice, a great place for training ship captains, as the seven seas and believed one should know how to sail in and out of the marshes before becoming sea going. The early Muslims wrote of the Seven Seas between Arabia and present-day Indonesia (a very popular trade route). And now today geographers and others will create unique categories of oceans and seas to create their own Seven Seas.
Perhaps it is the old belief that seven is a perfect/God's number. No one can say for sure but when various cultures consider the vastness of water Seven Seas is agreed upon as the great openness out there to be explored.