When I'm preparing for a swim, I imagine absolutely everything about it: the color of the water, how cold it is, the taste of salt in my mouth. I visualize each and every stroke.
The swim at Deception Island was by far the hardest swim I've ever done. Antarctica is a very unforgiving environment. If you don't train properly, you'll die.
When I was born, the world's population was 3.5 billion. There are now 6.8 billion people on the planet. By 2050, that's expected to rise to 9.4 billion. What's more, the Earth's resources aren't growing; they're decreasing - and rapidly.
Britain has bred many great explorers, but they seem to get so little coverage compared to soccer and rugby players.
Not to be too grandiose about it, but in a way I see myself like Sir Edmund Hillary. The water was my Everest.
On my second swim at Deception Island, the water was very clear and I was looking at hundreds of whale bones beneath me. It was a graveyard from the whaling some time in the 1920s-30s.
Of all the creatures in the world that really frighten me - the hyena in Africa, the great white shark - leopard seals are near the top of the list. They're killers. If my team spots one, they'll pull me out of the water.
The days of exploration of Shackleton and Scott are long gone. Everything has been climbed, crossed, done. Now what we're exploring are the full boundaries of human endeavour. It's not physical - it's all in the head.
I learned two basic lessons on Everest. First, just because something has worked in the past does not mean it will work today. Second, different challenges require different mindsets.
On 15 July 2007, I swam across an open patch of sea at the North Pole to highlight the melting of the Arctic sea ice.
The problem with climate change is that it's quite complicated for the ordinary person to understand.
I've been on swims where people have freaked out about sharks. You have to think about something else, otherwise it will absolutely paralyze you. I do math problems, anything.