Winter swimmers should stick to it from summer, and it is best to do it step by step. There is also to prevent hypothermia, do what you can, and enough is enough. The accident happened in our winter swimming club, the main reason is "personal heroism", not according to science.
The human body has its limits in water at 0-4℃. In addition, everyone's physique and cold tolerance vary greatly. The difference between young people and old people is even greater.
It is suggested to read the basic requirements of cold water diving in the US Navy diving manual before defining your own extreme cold tolerance envelope. The general principle is that when the water temperature is 0 degrees, men under 38 and healthy people should not exceed 30 minutes. Otherwise, you will have hypothermia, which may be fatal.