I'm not sure what else you'd use a rope for here.

My guess is it's a tow line for something. Notice how the rope is taught (would not be the case if it were an anchor, he'd have to ditch the anchor and use a new one if that were the case)

Though again it would be weird to have whatever you're "towing" that far below you, considering the longer the rope, the stronger the pendulum when wind starts to blow your gear around..