I am an electrician, I am an instructor of AC theory also, besides being Pontoon Beach electrical inspector. distilled water is a poor conductor but the impurities in the water and the chemicals make if a good conductor.... If the GFCI failed and a ground to the pump failed, the entire pool could be energized and the problem could be when they exited the pool. Also there is "stray voltage" that is another story... Buy even when everything is originally installed correctly,accidents happen. I hope they are recovering...