Do all these elite athletes and your son know that getting into cold water makes your blood pressure spike into stroke territory? If it doesn't kill you that's an indication you are healthy.
I used to do something like that. I went to this yearly pagan festival where we had a sauna next to a little lake. We'd all get naked and cram into the sauna.
Did tell you pagans love to get naked together?
Anyway, (Shut up. This is only a response so I get to do that) the truly cool among us got out periodically and jumped into the lake. There was a handy little dock there for that express purpose.
I must have been healthy. I survived to tell you about it.