I don't intend to mock people who don't know how to, but I am curious as to which it is more of. I grew up around water my whole life so it's second nature to me. But, I've met many people who say they don't know how to. The actual physical ability to swim, whether that be tread water or doggy paddle isn't difficult from an actual physical aspect, hence the question of is it a mental thing of being scared of water and fearing to try it.