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.