Oh fair, that is the first time you mentioned kids specifically. You've said/implied that it was simple and natural in every comment though so it was that attitude that I wanted to address.

If you want anecdotal evidence then here's this: I cannot swim and neither can anyone in my family. I don't mean we can't swim long distances or that we can't swim skillfully, I mean that we cannot swim or float whatsoever. I nearly died one time when we visited the community pools because I didn't expect the slide to plunge me downwards into the water. I got out by aimlessly flailing forward and getting lucky. And no that wasn't really swimming, it was more like walking on the floor while holding my breath as best I could. Also the lifeguard didn't notice.

At the pools, we always stay in the shallow areas that's waist deep. And the few times we've gone to the beach, we don't go further into the water than to get our feet wet. There are no other opportunities for us to swim here and even those 2 cost money. I hope you understand why it's unlikely for people in our situation to learn how to swim. Like, why take time out of my schedule to pay for something that I can't do, that my friends and family can't do, and that can potentially kill me?

For the record I brought up whistling because it's a skill that many people don't have. I've been able to whistle since I could speak and so could my parents, but it would be stupid for me to conclude that whistling is an innate human skill that all people know how to do. Likewise, some people just can't figure out swimming by themselves. The proper movement of their bodies does not come naturally and that's that.