I'm at the part where you stop the waterfall and drain the pool. There's an opening to push a block out of the pool down onto a switch below, but the path is to awkward and won't go through.:_(
put the stone with the face on it as far into the opening as you can, and then find the wall that has handholds carved into it. Climb up, hit the switch to fill the room with water again, and the water will force the stone out of the hole.