So let me get this right, water and electricity don't play well together
Well the company I work for is also the water utility & we get on alright!
I do recall that insulator pressure washing has been done in the UK at ground level, if the conditions are controlled (especially the water quality) it is not difficult.
From a helicopter as there is no connection or contact to earth the risks (electrically) are far lower, any electrical charge picked up by the machine is not a problem as the whole machine would be live at the same voltage.
(just as birds sat on powerlines don't get fried - same principle)