I'm always in the pool with my 3 year old. He's pretty confident in the water, but gets silly and loses his footing often. This makes him go under the water, and b/c it was a fall (vs. intentionally dunking) he panics and can't remember how to stand up, or gets too startled to correctly place his feet back under him. I pluck him out from under the water a few times every time we go swimming (every week to 2 weeks). (we're generally only in thigh high water or so)
If he's in his life jacket, he's allowed to swim away from me a little farther - but I'm still in the water with him. Without his life jacket I'm usually within about arms reach.
If I know for some reason I'm going to be distracted..talking, doing something - he wears his life jacket and is told to play out of the water for a bit. He can take it off again once I know my full attention is on him - he's very impulsive and a runner, so it makes me extra cautious.
I agree with staying close to our little ones in or near the pool. I felt I could relax a bit when they could swim the width of the pool. As I recall, that was around 6 or so but is different for all kids obviously. I was also always counting noses. 1,2,3,4 as we crossed the street or did anything else. LIke the Ducklings. And, I was also a lifeguard and while they can be good, they can't see everything so it's not safe to rely on them until our kids can swim by themselves. And, of course, there is no social pressure on earth that can dispel that fierce Mama protectiveness. Trust it.
I grew up with a pool and a lake in the back yard, and I don't remember any supervision. Also, I'm a very laid back mom. So it always surprises me at how anxious I am with my kids around water. I just have come to the conclusion that no one will watch them like a mom.
My good friend told me about how she had gone to a water park with a group, and when she had to use the restroom, she announced it to the group and said they needed to keep an eye on her 10 yo daughter. They all said yes, they'd watch her. When she returned, she was scanning the pool for her girl and finally spotted her- on the bottom motionless. The little girl was finally resessutated, but it's like my friend said: "She drowned that day. Just because she was brought back doesn't mean she didn't drown." Point of the story- never leave your kid with a group. If you must leave the child, appoint one person in charge.
My own story is that I was invited to go to a river with a group of friends. My beautiful friend who had organized the outing invited me by telling me that she knew how hard it would be to be by water with two little ones (18 mo's and 3.4 at the time), and she had already arranged for her high school aged neighbor to buddy up with the 3.5 yo. I was mostly paying attention to him anyway, and I didn't think anything of it when he came up on the bank to get a snack. Ten minutes later, the high school girl comes over with a sigh of relief and says, "I'm so glad he's here. I didn't know where he was." So the point of that story is be discerning about who that one person is who is watching your child. I can really only think of a small handfull of moms I would trust, and then only for a really short time.
At my pool, it is one (non-swimming) child to one adult. The adult must stay within arm's reach of that child. Any other non-swimming children in the family must wear lifejackets - the pool even provides these, AND they still aren't allowed to go in deep water. They will only call a child a "swimmer" once they pass the full swim test: jump in the deep end of the pool, and swim all the way across the pool and back without touching down, then climb out of the water without the ladder. It is a very strict test! I'm not even sure I could pass it, LOL. But, now that I've spent the summer going to the pool with my two non-swimming kids, I'm glad the rules are set up like they are because I don't have to fight with the kids about it (they are older and think they can swim, but they can't).