For what it is worth: I have my rabbits in an unheated shed. They use nipples similar to those used for chickens, except brass and spring loaded. I have a 16 gallon heated tub from tsc ($70) I put a circulating pump in it to circulate the water through the PVC pipe the nipples are in. There is about 60 feet of pipe. It seems to be working. I like I just dump water in the tub and don't have to fill bottles for 16 cages and try to keep them thawed. The tub is plugged into a thermocube.
I suppose something similar could be done with a chicken water system.
Love this idea, we have our rabbits in the basement this winter, but they'll be moved outside permanently come Spring. Filling up water bottles for just two rabbits is a chore and I'm not even dealing with freezing, outside temps yet. Definitely using this idea when we move them outside and start breeding them in the Spring. Awesome that it is workable even in winter. I think I spoiled myself with a once a week water change for the chickens (did I tell you how much I love my bucket? lol). Filling up water bottles for the rabbits every day is annoying! haha.
I never knew about submersible deicers... I think that sounds perfect. I'd like to add a heat lamp to the coop, but DH worries too much about fire hazards. I really need to get my butt in gear to properly seal up the drafts in the coop, the chickens seem just fine and the clear roof really helps to warm it up during the day, but I feel bad because it's so cold out there... I've never had outdoor 'pets' before so I feel weird leaving them out there.