Thanks to those who posted the CO2 method links. That seems like a godsend in difficult times.
That said, hypothermic methods have merit. As someone who has personally experienced several different "flavors" of hypothermia, it doesn't seem cruel. Hypothermia induced pain comes as you start to warm back up. Onset is much more ambiguous. What is scary is how PAINLESS it can be - suddenly you or your partner are making nonsensical decisions and slurring their speech. You get dumb, you get sleepy, go to sleep and don't wake up... Pretty universal across the animal kingdom.
The water thing sounds grim. If you immerse them over their head you are drowning them, not chilling. Now if the water was warm at the start, and one could just gradually chill it...