I was in Girl Scouts and we learned swimming every year at summer camp. Camp activities were often broken up by swim proficiency. So if I brought a good friend and wanted to do everything with her, but she couldn't swim and I could, our afternoons would be spent doing different things because she'd be in the beginning swim group and I'd be in the more advanced one.
As a kid my parents would always take us to the pool nearly every day in the summer. I never took official swim lessons, I just kind of knew how to swim because I was doing it all the time, and started out in the baby pool (I think maybe a foot deep at best, supervised at all times) and worked my way up from there. Our pool complex had the baby pool, the kiddie pool (three feet deep), the regular main pool, which started out shallow then went to a deeper end where you could dive. Separate pool for adults only (I think it was a hot tub type pool), a lap pool where half of the pool was sectioned off as lanes and half of it was open swim, and a separate pool with three diving boards. I had plenty of experience.
In high school we had a swimming pool in the gym. For basic gym classes there was a section on swimming, and you had to pass it. We had one girl in our class who had severe disabilities, both mental and physical. She was in the pool with us every day, but had a special para with her in the shallow end to make sure she participated.
We also each had to take a second gym course, but there was only one that didn't involve swimming. I took it instead of wearing the school's swim suits and using the school's towels for another day. Even the members of the swim team hated the swimming lessons in school because of that policy. Yes, they were cleaned between uses, but still...