Funny, I just took one and was considering posting this. It has an amazing effect of focus and relaxation. Seriously, nothing relaxes me like a cold shower it's like nature's Xanax
In March of last year, two weeks before I was due to move to a new apartment, the water heater in my old apartment broke.
Those two weeks of cold showers were some of the unhappiest in my life. The dread of standing naked in the shower with your hand on the knob. The involuntary spasmodic shivering when the jet of icy water hits your trunk. Panicked attempts to rinse off soap and shampoo that just don't want to foam in water that cold.
I used to be sick with sinusitis every single year(especially winters) for approx. 8 years. I don't even want to count how many packs of antibiotics did I consume during that time.
One September, I started to shower myself with cold water at least once a day.
It's been 4 years now and I haven't taken a single antibiotic since.
It also has a great mental effect - doing something a bit uncomfortable every single day makes it easier to do something else that's also uncomfortable.
Cold showers are a great way to start your day. It takes a while to get used to them but I've always found them refreshing. River immersions, I don't know.