If you regrounded the neutral down steam of the GFCI it would trip as soon as there was any current present in the circuit and this was most likely a fault from line to ground. Unfortunately water is not a perfect conductor so there will be gradients along that fault path that can give lethal results. My guess is the water was energized and the GFCI was bad or bypassed. If someone standing on that splash pad with wet feet reached into that water they would become part of that fault path.
I assume the safest thing might be to try to float until someone turned off the power. You want to be a "bird on a wire" and not create a fault path with your body.
I am sure there will be multiple violations uncovered if anyone actually looks at the whole installation but I bet the first one is someone jumpered out the GFCI because it was tripping. (or it was bad and not tested regularly).
I expect we might even see mandatory GFCI testing with logging and tracking after this. I am sure Morgan and Morgan is already there.
