I think you'll never find a case where someone got in trouble for not being a hero.
I've recently found myself in a somewhat related situation where a guy turned violent in a pub... first I tried to calm him down and almost got hit... he then turned to other guys who were nearby, and one of them got punched in the face and fell unconscious. My family was with me and told me to stay the hell out of it, but I thought that would be extremely cowardly so I jumped at the guy to try to keep him down, but he was strong and I got a punch in the eye which cost me a week with a black eye, but could've easily turned out much worse for me. If I had just stayed quiet, would I be "negligent"?? The police told me what I did was good as I was trying to help someone, but I didn't have any obligation to do it.
In the case of a child in a pool, the difference is a matter of degree. What if I am terrified of water myself? Does that justify my inaction? What if I just "froze", which is common in stressful situations. Does anything justify not doing something?