I personally don't keep water in the coop. It spills and heated water this time of year puts out a lot of moisture. The birds are provided an area with two sides protected from wind so are outside every day. In this way I never need to have food or water in the coop.
With a vent on each side of gable there isn't a lot of air being forced out if the coop was built well. Tight corners and overall good construction is actually a downfall as it doesn't allow good airflow coming into the coop. The air flowing out is limited by airflow in. If you've a gable coop then adding a few inlets on one of the low roof ends would provide intake that will run along the roof line mixing with the hotter, wetter air in coop and pushing out the gable vents. The roost can be around 18 inches lower than these intake holes or even on the opposite side of coop. Hole saws are a wonderful thing. Simple 2 to 3 inch hole saw and few minutes to pop out 4 holes along one of the low roof ends would stop the frost in coop. Hole saw is just a round saw blade you attach to drill. Cover the holes with hardware cloth to keep out the weasels and your good to go.
I like single slant roofs just because it makes venting easy. Intake on low end and exit on high end. With a gable design the vents there are only exits so some intake is needed unless you have a nice and shabby construction with good cracks in corners and around doors. It's not passive air flow, air will literally be sucked in and pushed out the top. Rate of flow is exponentially increased if vents are placed properly. It doesn't take a huge area, actually the larger area turns passive flow, smaller areas undergo natural vacuum forces.
