Karn - your thinking of the Scriptures of Abaddon found in the Temple ofthe Six Gods. And he didn't boil the water, he caused the stars to fall from the skies. Which sounds a LOT like what happened during the searing.
deco - While it's true that the desert would take a while to form naturally, all that's needed to form a desert the way we are speak would be waves to cause erosion. If in the battle the waves of water sped up *i.e., becomes hard and are more frequent* then the formation of the crystal would be new. Also, there would be some formation of the crystal desert with the very fall of Abaddon - as his fall caused the sulfurous sand in the Desolation.
The "Desert Searing" - which could also have removed a lot of water - was also done during that war between Abaddon and the 5 gods. And the area of the Crystal Desert is large enough to equal (or close to) the size that the Searing caused in Ascalon. And a war between gods can very easily increase erosion - possibly as an act of removing evidence of Abaddon they reduced the crystals in size to the point that a small desert was formed where there was water and island - back then larger crystals were probably more abundant. No one said that the Crystal Desert we know of now is what "instantly" formed.
And the "connection" between Abaddon and the Searing is that Abaddon gave the Charr the Cauldron of Cataclysm.