He wrote an interesting book called "Alone In The Ocean" [2] where he described his endeavor.
[1]: https://en.wikipedia.org/wiki/Stanislav_Kurilov
[2]: https://www.amazon.com/dp/B01HYLLOZ2/ref=dp-kindle-redirect?...
https://en.wikipedia.org/wiki/Gu%C3%B0laugur_Fri%C3%B0%C3%BE...
Not mentioned in that link though is that the doctors who examined him believed his resistance to cold came from him eating a traditional diet that included seal meat in his youth, and that his body fat had accumulated a not inconsiderable amount of seal blubber.
In that water he would have succumb to hypothermia in hours. I spent 2 hours wading in water with no protection in the same temperature that he was in. My body temp was 95 degrees, lower than his after 72.
this sounds dangerous. Just curious, why were you in 60deg water for two hours?
It's one thing to sit neutrally bouyant in a Scuba suit, or in a flotation device, with nothing but shivering to keep warm. Asleep, you can get hypothermia in water that is nearly at body temperature.
However, endurance athletes (like this 2.5-day-swimmer apparently decided he was) can keep warm by exertion. Competitive swimmers will keep warm, and actually sweat a great deal, in a very cold pool throughout an hour or two of practice in nothing but a speedo.
The problem only comes when one discovers, like the protagonist of Jack London's "To Build A Fire", that you lack the endurance.
Body Size and insulating fat at can also make a huge difference. People in warmer climates are simply less adapted to cold conditions and therefore at greater risk for a given temperature.
PS: Some people can actually walk around without clothing or becoming cold in 40f weather and be perfectly comfortable. It's sleeping that's the greatest risk as you reduce metabolic activity while having a large surface area in contact with the ground.
Incidentally, this variation seems not to be genetic, or maybe it is epigenetic. When visiting India during the "winter" with a friend who was born there but had lived in USA for decades, his family was shocked at how both of us were comfortable in short sleeves while they shivered in heavy coats.