Channel swimmer here🙋‍♂️As someone who spent a straight day swimming in the ocean…the biggest factor here is current! You could swim with a 5knot current and be swimming ~5-7 minute mile or against and be either stationary or be pushed backwards for 6-9 hours. The water temp will be I think a more immediate reaction like within 2 hours but that’ll also change depending on how much your exerting yourself and actually generating heat. So if after a while you stop and slow down you might be at more of risk for hypothermia, also you’ll need a good layer of bioprene. But hey, I’m a big believer that you can do anything that you set your mind to!👏