This is me. I will not allow CP to swim in a pool without one of us present. The one exception is if she is with grandma and grandpa and one of them is IN the pool with her. I have also allowed her to attend a swim camp, but there was always an instructor in the pool with her plus multiple lifeguards on duty. I also will not supervise any other other (elementary-aged) children in a pool unless I am in the pool with them or have another adult with me to supervise.
Here is a perfect example of why: I do allow CP to swim for short increments of time at OOT competitions. The pool we were at on Saturday had a few members of an IOC5 team in it. CP was so excited because a couple of them actually approached her (instead of the other way around) and asked if she wanted to stunt with them. I don't allow her to stunt with her teammates in the pool (that always seems like a head injury waiting to happen for me) but I was okay with her stunting with these adults because it was just a few of them in the pool at the time. CP was in heaven and I honestly think it helped her because she was much more solid and confident in the air than on day 1. Back to the story, the pool started to get crowded because other members of this team had arrived, so I told CP it was time to leave and to please go thank them for allowing her to stunt with them. She swam over to the deep end to say thank you and then turned around to swim back but there were two large groups of people stunting/hanging out around her. She tried to swim around them but couldn't find a path. I could tell she was starting to panic because she couldn't see the edge of the pool with all the people and couldn't touch the ground. I dropped all of the stuff I was carrying and ran over to the edge of the pool. I was about to jump in but when she caught a glimpse of me above the crowd, she took a deep breath, went under water and swam toward me until she found the edge of the pool. She was so shaken up when she climbed out. The point is, not one adult or any of the older kids she was hanging out with noticed that she was struggling. She is a strong swimmer, but that didn't help her when she started to panic. This just reinforced why I don't allow her to swim without myself or my husband supervising her. I am not sure that another parent or adult would have recognized that she needed help.