The Great Eastern Ocean was the main body of water to the east of the Kingdom of Narnia from its peninsula, later the island capital, of Cair Paravel. In fact, it was the only ocean in the Narnian world to truly be explored.
The Eastern Ocean consisted of three notable bodies of water, which included the Bight of Calormen that was located on the Narnian coast (also a nation/state where the Narnian Merpeople lived). Islands in this ocean included Galma, Terebinthia, the Seven Isles, the Lone Islands, Dragon Island, Burnt Island, Deathwater Island, Coriakin's Island, Dark Island and the Island of the Star.
Beyond these islands were more known bodies of water, including the Last Sea, inhabited by the eastern Sea People, beyond which was the lily-filled Silver Sea, the last body of water before the world's edge. Finally, there was the Utter East, a patch of land on the world's eastern edge that shared a border with Aslan's Country.
When the world had ended, it's inhabitants left their lands and entered Aslan's Country through the Stable Hill. The water levels rose significantly higher, and when Father Time had destroyed the Sun and Moon, it completely froze into solid ice.