Every single one of my kids has ended up at the bottom of the pool. My oldest insisted she could swim without arm bands at the age of 2. I told her to go ahead. She wore them after that. Ds13 ended up at the bottom at the Y - I took him to the bathroom, and he took off his swimmies, and family swim was ending, so I didn't put them back on, and went to get his sister out of the pool (turning my back to him for a few seconds), and the lifeguard was in the pool. The other 3 were at home, with tons of people around, and although they were being watched (and thank goodness, since it is soundless), it takes a second for a child to go into the pool (they forget they don't have swimmies on).
I also rescued a young boy from our pool club. His mom left him poolside to put her bag down, first day of the season, and he just walked right in, and under. Apparently, the previous season, he was scared of the pool, so his mom never even thought he might go in. Again, totally silent.
My kids had swimming lessons year-round since the age of 3, until they knew their strokes, and could swim the whole pool (50 meters?). By the age of 6, all could do this. I had a pool growing up, and I had year-round swim lessons as well.
All children should be able to be strong swimmers by the age of 6. Most of the drownings are toddlers who slipped in, or older kids who were not taught how to swim. Yes, it costs money, but this is one activity that is really important. Forget dance, gymanstics, TKD, baseball, etc., if you are short on cash, but get swimming lessons for your kids.
I had a 7 year old at a swim party go under, who couldn't swim (my dropped her off, and didn't tell me). Thank goodness I had dd14 sitting on a chair, staring at the pool, in a bathing suit.