This happened to me about 10 years ago. Changed me forever...
Big pool party with tons of kids. I was standing poolside watching as my 4-year-old swam.
His best friend, wearing swimmy arm things, tries to get out of pool and one of the swimmies slides off his arm. He slips back in the water and his mouth is 2" below the waterline. His eyes go as wide as saucers. There is no panic, no splashing, no noise....only his eyes. He cannot get his mouth above the water line.
I see this whole thing happen and it takes me about 15 seconds to realize what is happening. His dad is right next to him, but cannot see the kid's face.
I, dressed for a bbq with wallet and cell phone in pocket, jump in like a ninja, grab him and basically throw him to his dad.
People thought I was a lunatic until they noticed the kid puking up water.
I think of this several times a year during pool season. It would have been so so easy to not notice this kid going under.
Never, ever, take your eyes off the kids.