In 2008 I spent a week trekking in Pyrenees in Spain. I had to wash myself in mountain lakes and the water was sometimes 6 °C cold. But somehow I got used to it. When I returned home I couldn't stand normal warm shower and I had to use really cold water. After a while it returned back to normal but I took a cold shower quite regularly since then.
There are three places where I take a shower regularly in Stockholm: my appartment, swimming pool and gym. The showers there have either fixed temperature, or the temperature is not fixed but all you can get is 20-degree water. Which is too much. I remember that the best cold bath was in Soča river in Slovenia with 9-degree crystal water. I would say that this is the optimal value for me.
Fortunately, 4 Slovak friends were visiting south of Sweden in December and I joined them for the first 3 days of their trip. The very first hostel had old-fashioned showers and I was able to take a cold shower for the first time in 2 months. On the third day we were visiting Öland island with good beaches. 4 of us bathed in the Baltic see. Since it was cold (5 °C outside and 8 °C in the water!), we agreed that we are allowed to wear a cap but nothing else. After looking at the photos Marcel suggested that I should be disqualified because I was wearing my watch as well.
It was good but I have to say that I went out from the water too quickly and I didn't get that much endorphines. I have a suspicion that it was not 8 degrees cold, as I read somewhere on the internet, but a bit more.
If you are interested in taking a cold shower, I have just two advices for you. Do it quickly and use really cold water. Great description can be found in Ode to the Cold Shower Facebook group and also Google is always your friend.