The kid swim buoy are not life saving devices, even more when they have a seat with the holes for two legs. And that I guess at any age. I almost drowned when I was one or two, in the kiddy pool and in one of those, a bunch of teenagers/donkeys ran through the pool and that was enough to create a wave big enough for me to flip head underwater. My mom had to rescue me and remember this moment as the one the discovered how many insults she could say in english.
Also, around pools, what seems to happen frequently is that if there's bunch of adults and a bunch of kids, the adults will always think someone else is watching the kids. No one watch them, they get into shenanigans and accidents happen. So stay vigilant, if it's possible, close of the pool, remind them why it's important to have an adult present.
Here it's the law since 2006 that pools have to be secured, with an underwater alarm (that sounds if a kid fall into the water), a protective top (that can retain a child), a fence (with a height minimum) or an unclosed space. I can't find how much it helped with cutting down on drowning but they are sensible things to do anyway.
I used to think that it was weird that the first few times I went to my friend's pool (we were 7 or
her mother made me wear floaties and gauged my ability to swim before letting me take them off. But now I understand fully why. I had know to swim for a few years already but never thrust a 7 y/old on what he know, too much responsibility and life at stake.