Under water maneuverability works just like movement in the air. One or more props change speed slightly so the force exerted on the arm(s) increase/decrease and the whole unit rolls/pitches. The big difference it that underwater the blades are rotating much slower because of the density difference between water and sir.
That much was obvious, although how it knows it has broken the surface isn't, but I'm guessing that is related to measuring back EMF. I wanted to see how the actuators that changed prop pitch work, particularly the motors and gearing driving the two vertical arms that push/pulled the collar around the prop shafts shown at the very end of the video.