1. It was heavily hinted that she might have been saved by the Old Gods when the Sundering happened.
2. She was pretty damned close to everything that was happening under the Maelstrom.
3. Naga live underwater, they shouldn't care less what Deathwing does as cataclysm on the ground.
4. The first expansion with truly underwater questing, Azshara isn't part of it as an important character?