https://www.1843magazine.com/travel/taste-the-fear/what-it-feels-like-to-swim-in-subzero-waters

All I had to do was swim a single kilometre. It’s just that humans aren’t designed for immersion in -3˚C water.

What does it feel like when you first get into the water? It burns. It feels like you’re on fire. But in temperatures like this, you don’t die of the cold. You drown. Within the first five seconds your body goes into shock; it’s very difficult to breathe. The only thing I can do is count every stroke, “One. Two. Breathe. Three. Four. Breathe.” It doesn’t get any better after that. As the cold envelops you, it gnaws at your muscles and they start seizing up. Stretching one arm in front of the other gets harder and harder because you’re shivering – everything feels strained.

The swim took place in November 2017. I spent the week preceding it on the Falkland Islands, acclimatising. The water there is warmer: around 6°C. I started off doing ten minutes in the water, then 20, 30 and 40 to build up stamina. You’re only good in the cold if you spend time in the cold. Fat is a good insulator but so is muscle. Physically I try to look like a heavyweight Greco-Roman wrestler. I try to change my body to match the temperature. The colder it is the bigger I need to be, so I eat more and bulk up.