Lennart, you are most welcome to come swim Alcatraz with me whenever you choose
As well as anyone else who fancies it. I'll bet your body could take temps as low as 11 or maybe even 10C if you had time to acclimate. I believe a lot of it is mental over time. As Terry can attest, I'm not a large man (180 wet at best), but I can go long stretches in cold water with solid mental prep and acclimatization. Not fast yet but getting there, so if you are slow - people - you can last in cold water as long as you keep swimming.
PS EWA I'm spending Christmas in Perth, Australia where the temps will be scorching for me in the water (75-80F). I hope to one day swim The Molokai Channel. You up for doing it with me?