All I had to do was swim a single kilometre. It’s just that humans aren’t designed for immersion in -3˚C water. I was going to be the first person to swim in South Georgia (a remote island in the south Atlantic, east of South America and north of Antarctica), starting at the British scientific station and ending near the grave of Sir Ernest Shackleton, the polar explorer who led four expeditions to this part of the world, only to die of a heart attack during his last one.

If an untrained person were to dive into freezing water, death would come quickly. Even after all my preparation, as I braced myself for the plunge I knew my body would be fighting with all its might to stay alive.

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. I’m only wearing Speedos, goggles and a cap; I don’t grease my body for insulation like long-distance sea swimmers usually do, because if a seal or a killer whale goes for me, my team could struggle to drag me to safety.

For all its dangers, South Georgia is a magnificent place: full of wildlife, like an aquatic Serengeti. But it’s also a prime spot for commercial fishing. I took on the challenge of swimming in its waters to draw attention to this fragile ecosystem. South Georgia and the South Sandwich Islands are British islands – the British government has to power to save them.

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.

Freeze frame Lewis Pugh slices through the water

They say never do anything different on race day, that you have to do exactly what you’ve been training for, down to the finest detail. But this time I did something new. Most of the time, I’d dive straight in, but plunging into water this cold would be equivalent to parachuting onto the summit of Mount Everest. You need the steady climb up the mountain to get used to the lack of oxygen. So before the swim I paddled the route in my kayak. I got to know the contour of the bay, and where all animals were, while slowly getting used to the water. Then I dived in.

As I reached the last 150 metres of the swim, I went past a group of elephant seals basking on the shore. Six times the size of a polar bear, they’re slow on land but in the water they’re as slick as they are powerful. They could easily have killed me. All I could think was, “Oh Lord, please, please stay where you are.” I was going to stop if my safety kayaker, Dawid Mocke, yelled “out” because he’s spotted one lurking underneath. But Mocke had to keep calm. Fear is contagious and if he showed he was scared, I would have doubted myself.

I always say the bravest person is my wife because she watches me diving into such dangerous waters. When I reached the end of the swim I was bright purple - you could see my veins throbbing. I was disoriented and could barely speak because my tongue was frozen. It must have been scary for everyone watching, especially her.

In water that cold your blood rushes to your vital organs to protect them, but afterwards it flows back to your extremities, returning to your heart at dangerously low temperatures. This is why your core body temperature continues to drop long after you get out of cold water.

It’s not conventional practice, but when I get out of freezing water I try to get warm as quickly as possible. The first thing the crew did was rush me onto the ship where they put me in the shower. Not a warm shower, but as hot as I can handle. It feels heavenly. During every ice-cold swim I wonder if I’ll ever be warm again. As I sat in the shower, with my doctor watching, I began the process of warming up. It took half an hour to get out of the danger zone and about 50 minutes before I reached normal body temperature.

The whole expedition took four weeks of travelling from my home in South Africa for just 19 minutes in the water. From Johannesburg I went to São Paulo, then Buenos Aires then Chile: Santiago down to Punta Arenas. From there I flew to Stanley in the Falklands Islands, before a final three days of sailing to reach South Georgia. Returning took even longer: the boat had to sail back via Antarctica.

But I am willing to do it all again. When I did my swim only 2% of these islands were fully protected from the overfishing that threatens to tip the balance of this delicate ecosystem, which is already experiencing some of the fastest climate change on the planet. Now it’s 23%. But that means 77% is still vulnerable and I won’t stop until the entire region is safe.

Lewis Pugh, an endurance swimmer and UN Patron of the Oceans, has swum in every ocean in the world. He was talking to Caroline Christie