That is a huge shark, and it just keeps going around and around the boat like it is trying to find a way to get them, I am sure that actually being there on the boat with that big guy circling it, must have been creepy !
If they had been catching fish before, it is possible that the shark was just coming around because he had heard the other fish fighting as they were being caught, and was looking for a fish dinner. They said that the other fish stopped biting, so they had apparently been catching fish before the shark appeared.