Provided you came here looking for a shark repellent that could give you a 100% guarantee that it would keep you safe in the event of an attack, your search won’t prove to be fruitful. We have yet to come across a shark repellent that makes this claim.
One of the better ways to understand the level of protection afforded by these devices is to think of them as a seat belt. While seat belts reduce the risk of death in the case of an accident, they don’t reduce the risk of fatality to zero. Similar is the case with shark deterrents.