I disliked Vashj because the mechanics were clunky. Couldn't Blink on my Mage without blinking down onto the floor, my Warrior would charge shit and fall down for the same reason. You also swam alot slower in combat than you ran, so if you were casting or trying to strafe in-combat while swimming you felt gimped.
If they actually made underwater combat playable, sure. It wouldn't have been a terrible zone. But it was just painful, especially the first 6 or so quests until you got the Seahorse.