In this weather, the ferrets' water freezes overnight, and it never gets warm enough during the day to thaw again. Each day I've been bringing the bottles inside and running them under the hot tap until they're thawed - the ferrets love drinking the warm water afterwards. I suppose it's like me enjoying a hot cup of tea while I'm sitting in a chilly hide.
Then I had a brainwave - at least I think it is! I fired up one of the excellent Peacock handwarmers, and propped it between the ferrets' water bottles. So long as it doesn't go out overnight, it should provide enough heat to keep the water from freezing. I hope so anyway - I'll let you know if it works.
UPDATE: It worked a treat - the water stayed unfrozen, and was even slightly warm in the morning. On the other hand, Robin's simple commonsense solution (see comment below) is practical and cheaper!