According to Chilean website http://www.adnradio.cl/noticias/nacional/empresario-douglas-tompkins-sufrio-un-volcamiento-en-kayak/20151208/nota/3015494.aspx his body core temperature was 19 degrees Celcius (66 Fahrenheit) when he was admitted to the hospital. They managed to raise the temperature to only 22 degrees before he succumbed. Considering that normal body temperature is ~37.5 degrees, it's hard to imagine anyone can survive hypothermia this severe.

Neither site provides many details. General Carrera Lake is enormous, and I can imagine ocean-like conditions there, ie. strong winds and huge waves. I wonder what the water temperature was there at the time of accident - even though it is the beginning of antipodean summer, it could not be much considering the glacial nature of the area.
The average summer temperature in the city of Los Antiguos, located around the mid-point of the lake, is only 15 degrees Celcius https://es.wikipedia.org/wiki/Los_Antiguos#Clima, so the lake must be much, much colder.
Considering the conditions, I wonder if they were wearing drysuits?