I didn’t learn until late in life that you’re not dying of hypothermia in that water as I’d always been lead to believe. Your muscles get too cold to respond and you just drown. Horrifying.

Does regular warm clothing keep you warm in 15c water or are you referring to a wetsuit or equivalent?

Probably a wetsuit or survival suit are the only relevant clothing to help with a prolonged submersion situation

Your general point is totally valid -- cold water is dangerous. But your survival times for 15C are ridiculously pessimistic. 15C is just not that cold!

This source shows longer expected survival times at 0C: https://seagrant.umn.edu/programs/recreation-and-water-safet...

With training and when you are expecting to go into the sea yes obviously you can swim 20 miles across the English channel but all the cardiac arrests from cold water shock and general panic in the first 15 seconds bring the average down. The rule of 3 makes it easier to remember; they are approximate average numbers which are meant to drive the point home which is to wear a life jacket. We would prefer to find you shivering on a beach than face down in the sea.

This has nothing to do with trained cold water swimmers. Your figure is just completely off. 15C water does not kill people in three minutes. It does not cause cardiac arrest. An average person could last hours at that temperature. See the well cited link I provided.

Lying to people to try and encourage safe behaviour isn't wise. The truth is good enough.

he did say what brings that number down (it's an average). and yes, jumping/falling into cold water can lead to shock/cardiac arrest