Try to reach the room above the tower by whipping-up at the stairs near the skeleton, then proceeding until you finally use Azerim's Part to hover up.
There you'll find three statues, move them to the center hole as far as possible. This will give you access to a button which will raise the lightning bowl and reveal two more buttons which control the water level in the pool.
Use these buttons several times until you understand how they work. You must manage to get the pool half filled with water, this will finally enable you to access the tower and get Nub's Part.