I was lying with my wife next to the pool, casually watching. A small boy, maybe 6 or 7 years old, stepped to the ladder and climbed down into the water, in a completely confident, unspectacular way. With the water up to his neck, he did not start to swim, though, but just continued downwards, slowly to the ground of the basin. I loved to enter the pool this way as a little boy as well.
The boy made no movement underneath the water, at all. He just sank to the ground and that was it. There was nothing suspicious, and it took us some time to realize that he would not come back up. I would guess roughly two minutes. Then we and some people in the pool realized he was not moving for too long. The boy was quickly recovered, lifeguards rushed in and reanimated the boy. It was a remote location and it took the emergency doctors 15 minutes to arrive, in which the boy was puking, caughing and barking his soul out. His mother was just 10 meters away when it happened and had a complete nervous breakdown when they reanimated the child in front of her eyes.
So there was a happy end, but we were shocked to the bones that a child almost drowned in plain sight, 4 meters away from us, WHILE WE WERE WATCHING THE CHILD DROWN. The disturbing point was, that this child showed absolutely no signs of panic - he did not wave the arms, tried not to grab something, made no swimming movements with the legs, just nothing!
A few weeks later I was back in the same water park (which has a dozen of pools and slides) with a group of 7 boys in that age celebrating a birthday. I can tell you, I had no real fun that day, desperately trying to track the bunch and not loose them for a minute.