How common is this? In scouts we often did this, jumping into alpine lakes where you could see a glacier melting across the water.
We were also told that to treat heatstroke tossing someone in the lake and then treating for shock was somewhat preferable…
Ice hole swimming is common where I am, and each year some people die from fainting in the water due to entering too rapidly without training. A few years ago there was a case of an experienced woman losing consciousness and drowning in front of her husband and kids. Not sure about heart attacks in particular, I think you need an underlying condition to have an attack.
Very common.
There are quarries around where I am that have signs posted to not swim there in the Spring. Yet, like clockwork, 1-2 people drown each Spring.