However, I've heard a strong contrary opinion from a person who is admitedly a lay person wrt medicine:
All this talk about cold showers is an extremely dangerous and irresponsible fad! Do these people not know that sudden changes of temperature, eg going from a warm room right to cold water bear a risk of a cold shock, which may cause a potentially fatal cardiac arrest?!
I checked Wikipedia and indeed - from article Cold shock response:
Cold shock response is a series of cardio-respiratory responses caused by the sudden immersion of cold water (...) The cold water can also cause heart attack due to vasoconstriction; the heart has to work harder to pump the same volume of blood throughout the body. For people with existing cardiovascular disease, the additional workload can result in cardiac arrest.
However, googling for "cold shock shower" did not seem to bring me much interesting.
Some of what I was able to find is presented below. From the aforementioned MedicalNewsToday:
Some people should exercise caution when taking cold showers. This includes people with weaker immune systems and those with serious heart conditions, such as congestive heart failure. This is because the sudden changes to body temperature and heart rate may overwhelm the body.
There is a story of a woman who started taking cold showers and was experiencing what can(?) be interpreted as a mild cold shock in her first few days of the practice:
The worst part was breathing. Practically hyperventilating from the shock of the freezing-cold water, the sensations in my body felt similar to what I’ve experienced during panic attacks. (So, no, this is not fun at all.) About two minutes in or so, though, my breathing slowed down, and I was able to stand still and shampoo and condition my hair under the freezing-cold stream.
Finally I found an answer on Quora:
They [cold showers] are dangerous — you could go into shock. You could develop cold-related medical complications. (...) You aren’t about to do jumping jacks in your shower to prevent shock, are you? Thus, I recommend going down slowly. On day 1 you take a slightly colder shower. Keep it the same for a few days. On day you take a slightly colder shower, etc.
Based on all of the above my current hypothesis is that cold showers are a somewhat shocking experience to the body while it is not yet adapted, but are only really dangerous if one already has some severe underlying heart condition - but as you can see this hypothesis only rests on skimming over some not-necessarily-reliable (if not outright anecdotal) resources.
What is the truth?