hi, I have surfed and dived in Western Australia for many years. In later years I have used a shark shield and thankfully have not seen a shark when using it to provide a first hand testimonial. Spear fishermen have reported they have been of use whilst dragging a line of dead fish behind them but I can't personally vouch for that!!!!
Unfortunately we have had several fatal and near fatal shark attacks in recent years,several in areas that had never been considered "high risk".
The best "repellant" I think is precaution. Obtain local knowledge, do not enter the water around fishing activities or where decaying flesh may be apparent (this includes around crayfishing pots), be careful during early morning and later afternoon. Most of the attacks occured where one or more of these conditions were present. Always check before entering the water, large schools of fish and seals are a shark "magnet".
Sharks are magnificent creatures to observe if it can be done with safety.