The water used for insulator washing doesn't have to be demineralized, the conductivity just has to be below a certain level. Tap water is often suitable. There is a risk of the current passing through the helicopter (and occupants) if the gap between the conductor (wire), the aircraft and the ground (or tree), or between the aircraft and 2 conductors isn't sufficient. The spark will jump the gap, pass through the helicopter and jump to the earth or other wire. An Aussie machine got zapped a couple of years ago. Linesman knocked unconscious, pilot injured and aircraft damaged. A linesman in Australia was killed a few months ago doing live line work when the aircraft M/R clipped a wire.