From other boards, I have saw alot of people using the powdered Gatorade or electrolytes with success. But They always say, keep a bucket of regular water near, just encase a horse doesn't like it.
I live in Florida so usually its not that cold. But recently its been in the low 20's and I'm having to break the ice every few hours at night (I run on 3 hours of sleep from insomnia). Normally I just leave the hose running on low but it froze last night lol. I add loose salt and minerals into their feed which keeps them drinking. Also, if your concerned about colic, I add warm water and make their grain into mash. Haven't had any colic issues since July (that was hell).So I try to keep as much water in their systems as possible during the winter.