Gmr is right, The Crystal Desert *which also includes the Desolation* were the only parts underwater.
Basically, the whole desert thing, history wise, is this:
Crystal Sea separates Tyria and Elona
Crystal Sea is raised, Tyria and Elona are connected, Desolation is still non-existant *part of the Crystal Desert but not sulferous*
Abaddon goes against the other gods, his fall creates the Desolation *basically, Abaddon being put into the Realm of Torment caused the sulfer to appear, unless there was an event between the Crystal Desert forming and the Desolation becoming sulferous we do not know about.*
And in GW2...
Elon redirected by Palawa Joko, Crystal Desert *as a whole* gets seom vegetation.