I don't know about the effectiveness of Shark Shield - it is belived that this man wore it when he was eaten.
www.abc.net.au/news/2011-02-21/no-remains-found-from-shark-victim/1951028particularly read this part:
------------------------------------
Police are investigating if Mr Clarkson was wearing a shark shield safety device at the time of the attack. It emits an electrical wave to deter sharks.
The victim's friend Don Morrison has no doubt.
"I guarantee he would have been wearing it. I think he checked it was working all the time. I'm pretty sure he would have been wearing it," he said.
-------------------------------------
apparently he also wrote in his forum that he was always wearing it when diving.
Of course, since his body was never recovered there is no 100% proof, but somehow I doubt he would have just forgotten to put it on that one time.
However - the good news for those who worry about sharks - there are none at Boggy and Milang!!!