I always wondered how long a person of typical aerobic fitness could survive in a situation like that. Presumably you'd be pretty motivated to keep moving. If you're a world record setter, apparently you can survive for over 12 hours. (Record time swimming Monterey Bay) But I don't know if I could swim for 5!
I'm in OK shape and more importantly: swam competitively since not-quite-six through school, and then back in the pool 2-3 times a week after wrecking my back a decade or so ago... In 28C (pool, Caribbean) water I should be able to do 5 hours and some 7-10 kilometers, and come out not shivering too badly.
In 14C water: no thank you.