Well, I'm up here in New Yawk, near the PA border, and this one spot on the map is a zone 4, surrounded by zone 5's, and my koi and shebunkins survived just fine. Of course, the deep end is 4', and the middle is 3'.
I had a couple of iffy days trying to keep the hole in the ice, but this year I'll use a little bigger pump. I just set it on a shelf and bubble the water, no heater. The action of the water moving seems to keep it from freezing over just fine.