Seventeen hours in cold northern water without succumbing to hypothermia? My 'best guess' is he was not a small (read thin) man. I am thinking he must have had some 'reserves' stored as part of himself (chunky/chunky+/typical American these days)as an adipose tissue (fat) barrier to the cold water. Either that or he is an individual who is somewhat used to the cold. Cold toleration can be trained over time (NOT pleasant). An example would be SEAL training.
Anyhow here is one of those rare chances I get to justify the 'reserves' I pack around all day/every day. :-)

gimpy

Not to minimize the tremendous value of a strong 'will to live'.....
Likely it took both.