When it is that cold where I hunt you can usually limit 3-4 guys in a hour and a half or so. When the hunting is that fast paced the dog is almost always in the water. The water is pretty warm. When she gets out her vest seems to keep her pretty warm. I keep a close eye on her for any signs of cold. If she looks like she is getting to cold I will either run her around on the bank or toss a stick in the water for her to fetch. Both of my coldest hunts a couple weeks ago when it was -25 to -29 I never even saw her shake, never took her eyes off the sky looking for birds. Only problem she had was when we were finished and she jumped up on the front of the boat and laid down, then stuck to the boat. Had to scoop water up on her to un-freeze her legs and rear end from the boat. Just keep a eye on them and be prepared to pull the plug on the hunt immediatly if something is wrong.
I'm the type of guy who "cups" his hand over his butt, farts, and dishes it out like a soup kitchen.