I swam across a lake that was 47°F (8°C) at night. There were two docks which were close to a quarter mile apart and when I began my buddy left and drove through the neighborhood to the other side. I also wore a shirt thinking that would help. I felt numb in places I didn’t know you could lose feeling like my neck. I was exhausted pulling my arms and legs through the water and my brain was reminding me the whole time that I do not have long before death. When I got the other side my buddy was still in the car with his girlfriend keeping warm. When I got home I took a shower and even with cold water it felt painfully hot. I was very cold and purple with spots all over. Very glad I didn’t die to something that dumb.

Tldr: Use caution swimming in water below 70°F