OPEN water in the coop...
A closed system does not do this and as has been mentioned, it's nice to have water inside the coop during winter when the flock prefers not to go outside.
I keep a heated base fount waterer in the run and a bird bath deicer in a bucket with loose fitting lid on it with horizontal nipples for a heated waterer in the coop. Both have worked well down to -20F.
All waterers are best kept at chest height to minimize them getting dirty and the chickens...