I have. When I was in my forties I joined a swimming class at the local pool. The second week the teacher changed and, as the new one was noticeably pregnant she didn't come into the pool. She told us that we would float and that she would get us out if necessary.
We went in at the deep end, holding on like grim death and she asked us to try to touch the bottom with our feet, whilst still holding on to the rail. It was surprisingly difficult. Over the next few weeks she taught us how to float and that was the important thing. When you can float you can start to move about and, if you get tired you should be able to relax until you get your breath back.
I'm not a strong swimmer and the cold water, even in a heated pool. makes my wrists hurt. However, I enjoyed being able to relax and float and look up at the sky, for as long as I wanted to.
I heard a couple of years ago an instructor talking about the deaths that had occurred in flooded open cast mines. He said that people jumping into deep water got cold and so they started to thrash about which caused problems and caused some deaths. He said that they should relax until they got used to the temperature and then they could swim to safety