On the topic of safety, the Wikipedia page on ice baths states:
"There is agreement in the medical and scientific communities that ice baths can pose serious risks to health. Risks include hypothermia, shock and the possibility of sudden cardiac death"
I suppose everyone's needs are different, but if I was to advise Apple about what I wanted my watch to do should it detect that I suddenly plunged into ice water, it would not be to display the stopwatch and heart rate monitor -- rather I'd want it to call 911 and tell the emergency services operator that the wearer may have stepped onto thin ice and fallen into a frozen lake. 😀
Well there are risks with many things. I wouldn't advise jumping into deep water if you can't swim.
What they are referring to is 'cold shock'.
I certainly wouldn't suggest to someone to jump into ice water if you have never done it.
It's best to acclimatise, eg. warm showers slowly turned to cold and increasing the exposure time.
Your body is an amazing thing and will adapt. Of all the horrible things that can happen to you in life being in cold water for 60-90s (you don't feel it after that) is low down on the list and the benefits outweigh imho.
Some health benefits I have noticed personally :-
Reduced inflammation (DOMS, Sore back etc.)
Resilience to colds due to boosted immune system
Fantastic sleeps due to melatonin production increase from brown fat
Skin feels and looks better
Slightly slimmer (brown fat)
YMMV