It's the bait bag. Fisherman can buy live bait or "make bait", catch smaller fish that the larger fish eat and bait their hooks with them. These guys have a small boat and kept the bait bag in the water to keep the fish alive. The shark was after the bait. Bigger boats have a bait tank on board with circulating ocean water to keep bait alive.
Ah, so it was just a shark being a shark then. Either way, I am glad no one got hurt, the shark included.