As a child (8) i stepped down a very steep slope in one of the pools at the Sol Patos in Costa Del Sol. I can swim like a fish and could then to thanks to my mum sending me off for lessons.
I still nearly drowned, the steep step got me mid inhale, a huge gob full of water and you are under fighting as your coordination goes. Half choking and the other half attempting to self rescue. I managed to grab the side fortunately as my mum was on the other side in the shallow end nattering, she didn't notice a thing, just more splashing from her overly energetic kid as far as she was concerned. If i was not near the side i am not sure i would have got out of that.
Never told mum or that would be it, she would have demanded the pools and sea be drained immediatly. I coughed the water up and/or swallowed it and just carried on like nothing happened 5 seconds later.
I can certainly see how a non swimmer or even a not very confident swimmer could lose their life pretty easily. I was lucky i stayed calm, for the first 2-3 seconds until i got a hand on the side though it was a lot of fear. If they got a gob full in panic and were not near the sides i can imagine they died if there was nobody else around to raise the alarm.