Morgana had to let go of his coat, just for a second, to change the setting, when he's pulled under, inducing her first moment of true panic throughout this whole misadventure.

When the Doctor resurfaces, she reaches out and grabs him with her left hand, and points his might-be-a-screwdriver with his right.

She wants to close her eyes, but her training -- with fencing and a bow and arrow (the latter of which she is nowhere near as talented with as the former) stops her. When she presses the button, the whine is similar to the first time she saw it -- and the exploding rock which was terrifying at the time, but nowhere near as frightening as right now, if this does not work.