I think it very much depends on where they live. But part of the reason more people don't know how to swim is probably because many urbanites don't have good access to pools. Many public pools in urban areas were shut down in response to civil rights when black people were no longer allowed to be blocked from using them and racists would rather nobody has a pool than allow black people to swim with white people. And then in later generations, if a parent doesn't know how to swim, they aren't likely to take their own kids to go swimming.

A lot more rural people know how to swim because of all the natural bodies of water around, but our population overall has been consistently skewing urban for a number of generations now.