I know I'm a little late to this thread, but it reminds me of another thread where someone else was using a water heater in the bucket. In that other thread, the OP was wondering why his water heater element instruction said he needed to drive a grounding rod into the ground for proper use of the submersible heating element. Many people suggested that if he just used a GFCI outlet, he would not need a grounding rod. Sounded reasonable to me.
However, as I looked further into that issue, I read that these submersible heating elements can build up a difference in voltage in the water and the ground the chickens are standing on. Although it might not be enough to physically shock the chicken and kill it, the difference in the voltage affects "the taste" of the water because that voltage difference does bother the animals. That was confirmed by a horse rancher who reported that he did not have his heating element properly grounded and he noticed his horses would no longer drink from the water tank. Although they were not getting shocked by drinking the water, he was able to measure a difference in the voltage which turned his horses off from drinking that water. After he drove a rod down into the ground (as advised on the package), that corrected the problem. I won't pretend to understand the details of how all that works, but my take away from that discussion is that a small voltage difference that we cannot feel with a shock, can indeed be present and affect the "taste" of the water and turn the animals off from drinking from that tank.
If your chickens are not drinking from your bucket with nipples, whatever the cause, that would be a concern to me. I am also in the old school camp and use a 3 gallon metal water fount with a metal heated base to keep the water from freezing. So far, it has kept the water from freezing down to -22F at night. I really like the old metal water founts because I can simply look down on the waterer to see if the water is frozen and every morning I lift the tank up to feel how much water is left. I have a patio block under my metal heater base, and the water fount on top of that. So the water is about 5 inches off the ground. The water stays pretty clean that way, but if it does get dirty, I simply swish around the water in the fount and dump that small bit out. My 3 gallon metal water fount lasts me about 10 days for my 10 chickens. I'm very happy with that old school method as others have also mentioned.