Question: From: <http://sharkshield.com/>

Shark Shield is a unique shark deterrent system utilizing a technologically advanced three dimensional electrical wave form. Shark Shield technology is available for recreational users and water sports professionals to help all people enjoy the water with a new dimension – peace of mind; for themselves, their families and employees.

I'm referring to Shark Shield as an example. **What does a shark repellent circuit look like?** What would be the maximum range you get under water and does it depend on the power source? 

Is this simply putting + and - nodes under water and pulsating them with DC power?

Answer: 
There FAQ has a little more information <http://sharkshield.com/?/m/faqs> -
From what I gather from the limited info on the site, is that there is a small signal coming from the antenna. As for the specifics of the circuit and signal, its probably fairly simple, just a signal generator and an amp to the antenna. (just a guess)

The question is does it truly work, I have no clue im not a scuba diver or ever plan on being in the area where there are sharks present.

one interesting quote that gives some clue to the power is

> 
> Has the Shark Shield been proven to be safe to humans?
> 
> 
> The field generated by the Shark Shield, when used in accordance with the instructions >contained in the instruction booklet supplied with each unit, poses no danger to the >user, to sharks or to the environment. The field can be detected if the electrodes come >into very close contact with the skin.
> 
> 
> Direct contact with, or very close proximity to the antenna, may cause twitching of the >surface muscles of the skin, in time with the slow pulsing of the signal. The conductive >field readily travels through seawater, it being a better conductor than the human body. >Thus the field tends to surround the body rather than penetrate it. Scientific tests show that the type of signal generated by the Shark Shield is unable to pass through body tissues, unlike radio waves or microwaves that readily penetrate the body, and therefore it poses no health problems for users.
> 
> 
>
This sounds like a complete joke to me, but I will entertain an answer. Electrical energy does not propagate very well at all under water. The "skin depth" of water is something like half a meter @ 1MHz and maybe 7m @ 1kHz. As a rule of thumb, the lower the frequency (i.e. the longer the wavelength), the bigger the antenna you need to radiate at that frequency. As a point of reference, I think a 1kHz quarter-wave dipole antenna would be something like 25 miles tall. I would be more inclined to believe a shark deterent system (if such a thing exists) would be based on very low frequency acoustic technology... 

On the other hand, the article linked by @jsolarski above makes it sound like there is some actual scientific research out there that supports it experimentally...