Mariners have long used the phrase “seven seas” to describe the waters they sailed.
Originally, they may have been referring to the Arabian Sea, the China Sea, the Indian Sea (ocean), the Mediterranean Sea, the Red Sea, the Persian Gulf, and the West African Sea (part of the Atlantic Ocean).
In antiquity, however, the number seven was often used to mean many, uncountable, or all. Also, the word sea is commonly used in place of ocean.
When used now, the phrase “seven seas” seems old-fashioned and suggests a romanticized view of life on the ocean.