In spite of my apparent cynicism, I totally agree with Mitchbat.
I think they will make a big difference in the number of 'attacks' which are just inquisitive nibbles.
I also think it is highly unlikely they would have a negative effect, i.e., they would not increase the risk of an attack.
I have never met an animal yet that enjoys a whack from an electric fence, and considering how sensitive a shark is to electric impulses, it's highly unlikely that a shark would enjoy such an encounter, even though the energy output is very much less. Although dogs have very sensitive noses but still enjoy a good snuffle over a long dead animal. Beats the heck out of me. That would have to be one hell of a hit to such a sensitive instrument.

It will be a good thing if many people use them over an extended period so we can get some reliable info as to their overall effect.
If it turns out in 5 years time they are in widespread use and nobody gets attacked while using one, it would go a long way towards establishing their effectiveness.
So Mitchbat, hop out and get one and you can report back here with your impressions.