I agree that air temp is not the same as water temp. Contrary to some ill advised opinions, water temp here gets into the low 30's. The big problem we face with ponds is needing bubblers to keep them from freezing solid. Most people with koi either bring them into a house tank or to a LFS where they winter-over. Since the frost/freeze line is at 3 feet on average, most ponds are closer to 6' or more. It is also common practice to increase water flow in hopes of it not freezing solid. Not every pond makes it through a real cold winter and neither do some of the smaller streams. Just bring the temp down slow like seasonal temperatures do and you should be ok. If you know someone with a chiller for a saltwater tank, that would help. Trying to maintain a a cold enough temp will be a problem.
Many of the ponds I fish at in early spring are still around 35-38F water temps and even with wearing cold waders you can't be in it long. Some of them barely make it to 70 by summer's end. Every spring someone drowns because they fail to account for how cold the water actually is. By spring thaw the fish still alive are so doggone hungry that they hit at just about anything including a bare hook (happens but very seldom. You can usually fake them out with a shredded cigarette filter before they catch on. I have no idea how they manage to subsist, yet they do. Also very common to see ppl out on the ice pulling them in. Just wish we had crappie too. Then it would be complete!