PT6ER
There is a potential difference between the helicopter and the conductor or shield wire and may be up to a few thousand volts so the linemen need to be protected, but it is very minor compared to the voltage of the transmission lines which range from around 130 KV up to 765 KV. If you have seen the videos, you will see the arc from the wand to the conductor from around 12 inches depending on the line. The safety distance for phase to ground for an average transmission line of 230 to 242 KV is 5 foot 3 inches, a very different beast from static electricity. A 765 KV line has a minimum approach distance of almost 15 feet and a phase to phase safety distance of 26 feet! Drop a load of water across that line and it is going to be spectacular but it is an unrealistic scenario. For interest, a 765 KV structure is so large that if you try to land on the cross-arm on the top, the skids of a LongRanger are not long enough to fit across the gap between members. It is a very large structure and water drops are normally made from a lower height so dropping from overhead a 765KV line is unlikely to happen.
As the helicopter is, or should be, above the power line, the current will follow the path of least resistance which would be from the line, through the water to the ground leaving the helicopter pilot amazed at the violence of the explosion but otherwise unscathed. If you have thick smoke under the line and add a water drop to that equation, a phase to ground flash-over is a distinct possibility. I have dropped water near lines but have never had the need or desire to drop directly over any and wish to keep it that way.