The Last Sea was a near-saltless sea located at the south-eastern edge of the world of Narnia. It was the nation/state of the Sea People beyond the eastern bourder of the Narnian Empire, and was the area between which the salty Great Eastern Ocean transformed into the freshwater Silver Sea at the End of the World.
Caspian X and his crew aboard the Dawn Treader were the only known explorers of the Last Sea, located between Ramandu's Island and Aslan's Country.
When the world ended, the Sea People escaped through Stable Hill into Aslan's Country, and the waters of the Last Sea rose significantly and froze into solid ice.