Silver Sea

The Silver Sea was a body of water at the eastern-most end of the World of Narnia, whose surface was covered with white lillies. King Caspian X and the crew of the Dawn Treader encountered this sea on their voyage, originally naming it the Lily Lake.

The Silver Sea was the closest body of water to Aslan's Country, with the lilies extending right up to the standing wave that separated it from the World of Narnia. To undo an enchantment that was holding three of the Seven Lost Lords (Argoz, Mavramorn and Revilian) in eternal sleep at Aslan's Table, Reepicheep the Mouse voluntarily rowed his small boat up this wave and entered Aslan's Country, vowing to never return to the World.

At its southern edge, the Silver Sea met a grassy plain, at the end of which the sky came down and met the surface of the earth.

When the world of Narnia was finally destroyed by Aslan, the part of the ocean nearest Narnia rose significantly and froze into solid ice. Presumably the Silver Sea followed suite and froze as well.

Trivia

 * The Silver Sea became shallower the further one advanced into it. Even before reaching the standing wave, it became so shallow that the Dawn Treader could not advance without becoming stuck.
 * At its easternmost edge, where the lilies met the standing wave, Edmund, Lucy and Eustace could easily walk through the water.