This story is being told second-hand by a non-specialist, so some minor details may be wrong.
My best friend worked as a swimming instructor alongside his mum several years ago, teaching young children. Due to the children’s age, at least one parent or guardian was required to be present for the entire lesson.
One day, in the middle of winter, [Friend] entered the pool area with a gaggle of kids and parents and immediately noticed that the temperature of the room was way lower than he expected. Sure enough, the pool heating had failed overnight, and the pool temperature had dropped down to 20C (68F).
20C might sound nice and warm, but as water is a very good conductor of heat and has a very high heat capacity, that is actually quite cold water. It’s not icy cold but enough that you’d need to acclimatize to it before being comfortable swimming, and way too cold for small children and toddlers to safely swim in.
However, a 20C pool will keep the air at a nice and warm 20C, which feels even warmer in winter, so some of the parents didn’t believe [Friend] when he announced that lessons were cancelled for the day due to the cold pool. One father in particular took issue with this and demanded that his child be allowed to swim, but my friend held firm and said no.
At this point, [Friend] turned around to speak to some other parents about refunds for the lesson and reschedules for the more advanced swimmers. Then, they all heard a splash coming from the pool.
Fearing that a child had fallen in, [Friend] quickly turned around and was greeted by the sight of a grown man, curled into the fetal position, rapidly sinking into the deep end of the pool. The angry dad from earlier had wanted to show my friend that 20C was nice and warm, and of course his son could swim today, so he’d stripped to his underwear and dove in.
Here’s the thing about cold water: if you jump in and are not used to cold water, your body gets so overwhelmed by the sudden cold shock that your muscles all cramp up at once, leaving you to sink like a rock. 20C isn’t massively cold, but it will feel like it if you’re standing in a warm, humid room and expecting warm water.
At this point, [Friend] knew that someone needed to rescue this guy before he earned a Darwin award in front of a dozen small children, and knowing that his mum wasn’t strong enough to pull a full-grown man off of the bottom of the pool, [Friend] dove in himself.
Now, [Friend] is hardly immune to cold shock, either, and knew he only had a few seconds, but a combination of keeping his shirt on to give him a slight edge in insulation and expecting the sudden shock of the cold water allowed him to dive in and drag the terminal idiot to the edge of the pool, where the other parents hauled him out and got some towels on him.
[Friend] only managed to also not drown by holding onto the ladder, but he told everyone to make sure that the parent was dry before he got out. He had to be helped out by his own mother, along with a few other parents, and he needed the rest of the day off.
The moron who started this entire thing was banned for life for disobeying a lifeguard and endangering staff, but it was made clear that his son was welcome back, so long as he was accompanied by a competent adult.