Seriously it sound odd but the number 1 defense thing you can do is hit the sharks nose as hard as you can or hit him in the eye. Sharks have an intense amount of nerve endings and EM field detection in their nose its how they sense migration paths and feel their way through the water Hitting it hard in the nose is the most common deterrent told to ppl by lifeguards an such as to the shark its essentially like gettin kicked in the nuts