I used these when in school for diving on Tangalooma (worst danger there was the wobbies) I turned one off, dove next to one on the bottom and turned it on and it took off quite quickly so I can say they work for the small ones at least. Only annoying thing is if the antenna touches you or anyone else they get a nice zap. This was a few years ago so they may have fixed this.
http://www.sharkshield.com/buy/freedom7/GU1000A