Fish, in water with rapidly falling temp. is hardly the same as putting a live bird in a freezer. i don't advocate tossing a fish into a freezer to flop around and freeze solid to the side of the Dove Bars.
I got the to the early stages of hyperthermia about 10 years ago. I got soaked while Elk hunting at 11K ft. I managed to keep my wits and hike just over a mile to camp. When I stumbled in I was shivering, exhaustion and starting to get confused. Fortunately, my friends we in camp to help. Dry cloths, warm water to sip, and I'm here today to take about it. I can honestly say I had no feeling of pain. I was/am experience enough to have know to force myself to stay calm & focused.
In 2004 I fell in a lake while (foolishly) ice fishing in November. The water was about 38 degrees(there is the foolish part...). It took my friends about 4 minutes to get me out of the water. It was another 4-5 before we got to shore, another minute or two of fumbling to strip naked and get in a truck with the heat on and dry blankets. It was about 10 minutes to the hospital and my core temp was 94 degrees--still hypothermic despite 10 minutes of active warming.
The only point I experienced pain was during warming. The shock of falling in the water and the near drowning experience scared the bejesus out of me, but it didn't hurt. As a matter of fact, after being pulled from the water, I remember basically none of it. I'm sure my recollection of being in the water is abbreviated. My brain was both hypoxic and hypothermic, so you're not really able to comprehend anything, even basic things like pain and temp.
I'm not advocating this method of euthanasia, but I think people have general outrage to things without generally understanding all of the science around it.