Have two water containers, and alternate which is in the coop freezing and which is in the house thawing. (I've done this with chicken waterers and rabbit water bowls, so I assume it would work equally well for any similar containers. I have no personal experience with ducks, just with water + cold.)If anyone else had any thoughts they'd like to share (especially about keeping duck water thawed in winter!) I'd love to hear it
Or maybe dump it after dark, and refill in the morning. (Works with chickens that sleep all night, not with rabbits that are awake and active all night. How well it works for ducks would depend on whether they want water in the night.)