The Great Sea is the setting of The Legend of Zelda: The Wind Waker.
This is the name of the ocean that flooded the ancient land of Hyrule. It started out as just a seal, but became a real ocean by the end of Wind Waker when King Daphnes Nohansen Hyrule used the Triforce to wish for the old Hyrule to be washed away, taking Ganondorf with it.