I use a stock tank deicer. I found one that is OK on plastic and is OK if it runs dry (is no longer submerged). I put it in a black rubber feed pan (used for horses).
Nothing else works for me because my electric goes out so much. That stick tank deicer can thaw itself out of a solid block of ice pretty quickly.
Never put the waterer in the coop area, it will increase your humidity too much and so increase the frostbite risk.
When it is warm here, I just have the kids take out water 3x a day and save on electric. The chickens will peck through a thin ice layer to get water. They aren't very strong though. When the water freezes solid in an hour, then I switch to the deicer.
You do have wide perches, right? I have found that it is NOT just if the belly feathers cover the toes, it is if the toes bend. The toes must be flat, bending the toes reduces circulation, and so increases frostbite.
@3goodeggs
of course, yesterday I ate a hotdog after it rolled onto a public picnic table.
I battle all winter trying to make sure that no pipes freeze (under houses, between houses, in fully insulated and heated pump houses, in the walls of my house 
)
.... But, it is vital that those chicken feet are flat... Trust me on that one. A little frostbite on combs or wattles is not a big deal, but you do NOT want any frostbite on the feet. 
Checking in. Nice pics all! 
) Actually at one point I was rolling on the floor just not laughing too much. 
poor bears.
