The Crystal Sea was what is now taken by the vast Crystal Desert, before it fell dry. The Margonites and the Forgotten used to sail this sea. Ship wrecks can still be found in many places of the Crystal Desert, far away from the current shores. The Margonite Temple of the Six Gods stood on the shore of the Crystal Sea before it was drawn into the Realm of Torment.
The sea is described in the Scriptures of Abaddon:
And so it came to pass that Jadoth, being persecuted by the horrific Forgotten armies, and hounded from his home, did seek refuge among the cooling mists of the Crystal Sea. Untold weeks passed as Jadoth huddled in his sanctuary, with nothing to see save the endless ripples of the boundless ocean.
On the 51st day of his exodus, a frightful sight manifested before Jadoth's eyes: the unmistakable shape of Forgotten warships upon the horizon's shimmering edge.
And prayed Jadoth, "Abaddon! Lord of the Everlasting Depths, Keeper of Secrets, open mine eyes and bestow upon me the knowledge of the Abyss that I might smite mine enemies and send them to the watery depths!
An unsettling silence swept across the waves. The twilight sky shattered and stars streaked down upon the Forgotten armada. The seas boiled and ruptured, and gave birth to a maelstrom from which not even light could escape, and transforming the sky above into a midnight void.
And thus was magic gifted to Jadoth, chosen of Abaddon, the first of the Margonites.
-- Scriptures of Abaddon, 1BE