i grew up in the Midwest (in Ohio) and was about 5 when i learned how to swim. took swimming lessons for a few years at a local pool.
for the longest time, i always assumed that *everyone* knew how to swim (esp after moving to California where pool parties, beach parties are so common). i was so shocked when i went to MIT for graduate school, and i met so many students who did not know how to swim. i guess i noticed because MIT requires its undergraduates to pass a swim test in order to graduate--you have to swim 100 meters (4 lengths of the pool i think) without stopping or touching the ground, etc. i remember the PE department had to hold tons of classes just for those who did not know how to swim at all, and needed to pass the swim test.
that being said, i am glad i learned to swim as a kid, b/c nowadays i pretty much hate swimming in general, and would not have the patience to stick with it and learn. i'm on the thin side, with skinny legs and arms so i always feel like i have to work like crazy to keep myself from sinking. when i was a kid i was a lot chubbier and i think that is what helped make learning to swim easier, since it was easier to float. plus, back then i don't think i was as easily bothered by water in my ears (nowadays that bugs the h*ll out of me!)
and to answer someone else's question-- CA does not have rule requiring to know how to swim before graduation. my high school didn't, neither did my brother's. it would be a good rule to enforce though, IMO. i think the issue is (like someone else mentioned) mainly funding, b/c if they impose that rule then they have to provide swimming lessons, etc, which cost $$$