I'm from MN, where it can get very cold (these past few days have been brutal - we got up to 0 or 1 degree this afternoon for the first time in almost three days!). My ducks (Anconas) use a heated bucket, and I've never taken it away at night. They dunk their heads in and splash around even on cold days, which ends up with frozen water droplets on their back and in their tails when it's really cold. Silly duckies! It looks awful, but they seem to be okay with the ice on themselves.

We actually just moved them into their new coop, which is insulated and which we have been heating on the very cold days/nights, and there's no more ice on their backs, which makes me happy, but they survived all last winter with the ice and came out fine. Ducks are well-built for the cold. I'm definitely also a overprotective mother about my babies! We spent an extra half-hour on the new coop (at midnight) because I wanted to make sure we dulled the little screw pricks on the underside of the new duck ramp.
As for the buckets - the one I use is the 9-quart, I think from Allied Precision, and it's done a great job at keeping the water liquid. The ducks splash water up on the edges, which does freeze into big mounds of ice that I've had to knock off, but it went down into the below-zero's earlier in December (before the ducks moved into the warmer coop) and the bucket (and ducks).
Hope everybody and everyduckie's doing okay with the cold temps! It's getting a little tiring when just going out to feed the ducks I need to pile on the snow pants and the boots and the coat and the gloves and the hat...

Please come back summer!