> cold showers can be dangerous for people with per-existing heart conditions
I think cold showers trigger the diving reflex[0] ("most noticeable effects are on the cardiovascular system, which displays ... in humans, heart rhythm irregularities"). I don't have a real source for that, but I tried cold showers a few times and found it impossible to breathe when I had cold water spraying on my face, like I could feel my airway closing involuntarily. Warm/hot water doesn't do that.