Generally speaking, the biggest problem with dropping water on energized powerlines is the possibility that the water will act as a conductor between the helicopter and the powerline. I have dropped on (over) powerlines many many times, and it has never been a problem for me, but I have always dropped from a fairly high altitude and made sure that the drop was cut off well before the bottom of the drop reached the lines. Having said that, I know of at least one collegue that has received a jolt from a drop over energized lines.