A dear friend of mine goes for cold-water swimming, which is good for you?

Yes indeed!

Kathleen McDonnell writes about the pleasures and science of cold-water swimming: “For centuries, claims that cold-water swimming improved human health and well‐being were based largely on folk wisdom and anecdotal accounts. That began to change in the late eighteenth century with the work of Scottish physician James Currie, who used cold water in the successful treatment of a contagious fever in Liverpool. In 1797, he published an influential pamphlet, Medical Reports on the Effects of Water, Cold and Warm, as a Remedy in Fevers and Other Diseases, that contains the first English record of clinical observations using a thermometer.

My regular is cold shower. Gets the day started right.