My old Cody boy would swim in freezing water, come out and have icicles hanging from his feathers. My rule is no swimming if there's ice on the surface of the water; otherwise, I let them and turn up the heat in the car after toweling well.
Now that Daisy's getting older, cold water really aggravates her arthritis. She's almost 9 so usually by October, the water's too cold (we're in Pennsylvania). But when she was younger, it didn't matter and I found she could decide herself when the water was too cold.
I think the important thing is when it's really cold, they need to keep moving when they get out of the water if they're going to stay outside. And I've also heard you don't want them to be standing still in very cold water either. Movement is key.
I think the best rule of thumb is leave it up to the dog UNLESS there is any ice on the water then it's definitely STAY OUT / STAY OFF!!!! Once a dog has broken through the ice it can quickly become a deadly situation.
One of my fly-fishing quests has been to hit "ice out" in high mountain lakes. This is when the ice in these lakes first breaks up in the Spring and as described by Gary LaFontaine, a very well known fly-fisherman, any fool who can throw a fly 15 feet can consistently take the largest fish in the lake cast after cast.
The key is being able to time the break up and most of the time after hiking in you find a lake still frozen solid which of course makes it difficult to fly-fish. I tried this many times with Sammie my first Golden and although the hike in was always the high light of her day once we hit the lake you had to be very careful. When I got to the lake I'd have her do a sit /stay, which she was very good about, while I checked the lake out.
I recall one lake which was only partially broken up and I decided to fish it when all of a sudden who do I see but Sammie sitting on a sheet of ice 15 feet in front of me looking at me with that smile of hers. I called her off it right away and thank god she came wagging her tail like what's wrong dad...this is fun. Moral of the story is you can't be to careful when you're dealing with ice and dogs.
I say never! I do agree with the ice, although in some streams and in our pool, the edges freeze over last and my guys will just swim in the water at our pool stairs (supervised so they don't go on the ice itself) or at the edge of the canal. Then they have icicles until they warm up, thaw themselves, and get dried off. But throughout it all, their skin never really gets wet. They are designed that way with their undercoats and oils.
I live in Maine and my boys swim when ever they want. I go to the beach 12 month of the year and let them decide. If it is very cold they tend to stay out of the water. If it is sunny and above freezing they will jump right in and come out and roll in the snow then go back for more. I just keep a close eye on them and watch for shivering. If they start to shiver I head back to the truck and turn on the heat.
To be taken with a grain of salt, but a more concrete rule of thumb I've heard/seen in the past (sources almost certainly include but are not limited to GRF) is that water temperature plus air temperature should be 100-110 degrees fahrenheit or greater. I'm sure there's some variance based on the individual dog though. One of my dogs does fine swimming at 100 combined or lower, whereas another seems to get shivery if the the air+water temperature isn't at least 110.
It's too cold when the water is completely iced over.
As long as you don't force your dogs into the water and it's not bitterly cold (which if it were, the water would be completely iced over probably) - the dogs should be fine swimming when it's 30+ degrees out.
^ This was December and it was about 50 degrees out. The water was much colder than that - and there was bits of ice out there. It wasn't ice covered though so safe for the dogs.
This was a February-March swim, I believe The river had sheets of ice in there, but the dogs did not venture out too far. Actually the youngest one chose to stay out for the most part. The dogs aren't stupid. They won't go in if it's too cold for them. Unless you are really pushing them to go in.