[panjumbie]

The issue of ESD is a very serious one. A couple of clarifications of previous posts:

To Fatlazyless: A GFCI breaker or outlet does not require a properly wired (or any) ground wire to function. In fact using a GFCI outlet is one of the few ways approved by the National Electric Code of connecting a three prong (grounded) 120V outlet to a wiring system with no safety ground wire, such as old two wire Romex with no ground or really old knob and tube wiring you still might find in a farm house.

Now obviously, some unqualified person could come up with a way to "connect" the GCFI so it never trips, although the design makes that difficult. And the early GCFI's were notorious for failing to protect, particularly after a nearby lightning strike or other surge. Newer devices are supposed to be more failsafe. But that is why the "test" button is there.

The GFCI senses the difference in current between the hot conductor and the neutral conductor. Normally they are equal. However if there is a ground fault leakage current, some of the current is flowing from the hot conductor to the ground leak and not through the neutral so the currents are not equal. This causes the GFCI to trip. Personnel protection GFCI's (class "A" devices) will trip with a leakage current present of 4 to 6 mA (that is 4 to 6 thousandths of an amp) if they are functioning properly. Since they sense the difference current they do not require a ground wire to function.

Be aware that online sources (not verified) claim that most humans can feel a "tingle" when immersed with as little as 1 mA flowing through their body (not enough to trip the GCFI) and as little as 10 mA is claimed to result in muscular paralysis, probably resulting in drowning. Not a lot of safety threshold between the trip point of the GCFI and the paralysis point.

To Ishoot: the current to trip a ordinary 15 or 20 amp circuit breaker "quickly", a fault current as opposed to an overload is in the range of hundreds or thousands of amperes. To get these kinds of currents to flow through "fresh" water, the immersed hot conductor and a metallic ground path would have to be quite close together. There are many, many cases of ESD where investigation has determined that the electrified immersed metallic item was probably electrified for a very extended period of time without ever tripping a non GFCI breaker. An ordinary breaker will carry 110% of its rated current forever.

One of the reasons why ESD can be so dangerous is that it is not uncommon for the fault to be intermittent in nature. Particularly on larger craft that have many 120V (or even 120/240V) devices. For instance, the heating element in a hot water heater faults to ground (a common failure in all hot water heaters) or the element in a stove similarly faulting. Only providing current when it is on. Or a defective lamp or light fixture, or a portable cord that is pinched to a metal deck plate. Only live when plugged in, and even then possibly intermittent.

As a licensed Professional Electrical Engineer (now retired) I'd say the only safe strategy is to keep 120V electric wiring away from your shoreline and don't allow anyone in the water anywhere close to any wiring that is or could become energized. Electricity and water just do not mix well!

Regarding the detection device, as others have pointed out, by the time any such device has alarmed, it is too late for those already in the water. The total lack of any listing by any recognized testing laboratory (UL, etc) is a red flag to me. And their own webpage says you should stay out of the water when electric devices are connected, alarm or no alarm.