After watching the video several more times, I think I figured it out. There is a VERY BRIEF segment in the middle, when he is surfing past the other surfers on their boards, that you can actually see a "wake" and churned up water ahead of the shark fin. This can only be a boat, which is towing a device, and the surfer's rope(rod) is tied to this device.
Still, I think it was a cool video. And he was still pretty brave (or stupid) to jump in off of a pier full of fishermen with hooks in the water.