[buildbot]

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…

[PyWoody]

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.

[sigmoid10]

That's a different kind though and only mentioned on the side in the linked blog. If you don't know what you're doing and you jump into freezing water for the first time, chances are that you'll breathe in water. Happened to me as well. Let me tell you, it really sucks. If you're unlucky, this can definitely be enough to make you drown. But with training you can easily overcome this problem. I have jumped into arctic waters many times since then with zero issues. What the blog claims, is that there is a vague, general physiological risk from some kind of mixed response targeting your heart. I have to say I've never heard of this before (except for people with pre-existing conditions) and the blog does a poor of explaining the biology beyond high school level anecdotes. It also cites zero sources. So while there is a risk, it is not what you might think after reading OP's link.

[PyWoody]

No, I am not talking about people drowning from the gasp reflex.

Every single year, during the first "hot" day of late Spring/early Summer, kids will get on their bikes, go to the local quarry where they've swum a million times before, and die from exposure.

The quarries are deep and surrounded by trees. Most of them don't receive any direct sunlight. So, while the ambient temperature may be 75F, the quarry water will still be near freezing. A kid will jump from up high on the quarry ledge down into the water, go into shock, and die before they can swim to edge.

Again, this happens every single year.

[orbital-decay]

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.