I would never consider having a cold shower or submerging yourself for a short duration in cold water to be life threatening! Just because something isn't comfortable, doesn't mean it's a terrible thing. I certainly wouldn't want to stay in waters near freezing temperatures for more than 5-10 minutes, but our bodies have many survival tools that kick in when it feels threatened. It activates our fight or flight response, which is a good thing! We are designed to survive.
Having said that, you have to do things like this with some sense. Don't just go and dive in a frozen Lake if you are new to cold exposure. You'll probably gasp for your breath the second you hit the water, inhale a lungful of water and drown! But when you carefully enter the cold feet first, you'll gasp for air and your brain is gonna get a threat response and think you're life is in danger and are going to die. But then you realize that this is just what cold feels like, it's not death. You will slowly become more comfortable after a minute or so.
If you're an old, unhealthy individual definately talk to your doctor. Although I doubt their textbooks have anything on the matter. There are just way too many positive feedback from cold therapy to deny that it doesn't have potential for more scientific research, which honestly, there is already plenty of already.