If they're getting thrust from their feet, why would they move them at all? Every move would turn them I should think.
I also believe there was propulsion from their boots, like in the old Marine Boy cartoons. I immediately thought of Marine Boy in the theater when I saw the trial of bubbles.
Small movements of their limbs would enable them to maintain a straight line of travel. The Human body isn't a nice regular shape, we're "bumpy" in form. Not moving their limbs at all could have resulted in a gradual turn.