Between these 2 steps:
"With either a lit arrow or the fire spell, now, burn down all the banners/tapestries. Two of them will lead into secret rooms (one on the left, one on the right). One of them (the one near the lever to your left) will contain a crystal.
Return all the way back to where you found the dead guy at the beginning of the watery depths. Before you reach him, look to your left. There is a path into the darkness going down. Go."
...you should put in that you need to spin the pillar where you burnt the banners. You have to do this to lower the water to do the 2nd step here.