I agree, I was in a swimming club as a child and can do all the strokes including butterfly, but that was all in a heated indoor pool and it's only as an adult that I've learned about how people gasp, swallow water and be well on the way to drowning if they jump in cooler (not necessarily cold) water.
Also the dangers from an acquaintance who is a water rescue specialist in the fire service. However, I'm far less likely to take part in risky behaviour of jumping in, where people get stuck in mud at the bottom of a canal/river, injure themselves on obstructions below the water, get swept away or find they're unable to get out again, but that's what people need to be aware of, ideally learning from an early age.
I saw a poster recently, not sure where it was, but it was the #beamate from page 5 here, encouraging teens to speak up if they see friends taking risks around water. This sort of stuff needs to be prominent on social media, ideally as unskippable adverts.
Download.ashx
It was drowning prevention week a couple of weeks ago so hopefully the message has got out there and people take notice before the next hot spell.