The problem as explained to me is not one of risk to the helicopter, but when the water hits the lines, it shorts with the ground and trips the grid. This can lead to damage to some of the network and takes time to reset. I guess this could be a problem if you are relying on electricity to power your home grown fire fighting system or provide power to the mains water pumping system.
I have bombed over power lines in the past before I was aware of the problems and never even noticed. The best idea is to get the power company to isolate the lines whilst you work in the area, probably only take a week or so
Heads down, look out for the flack