This is no joke, it is a real danger of which one must be aware. I have seen such a case. Several decades ago, the son of a coworker of my father, a young male

This is no joke, it is a real danger of which one must be aware. I have seen such a case. Several decades ago, the son of a coworker of my father, a young male in his twenties, not previously known with health conditions, has died by jumping in very cold water during a hot summer day. It happened that there under water there was a cold current and the water was much colder than expected. He had an instant cardiac arrest and it passed some time until people around realized what happened, so resuscitation failed.