Of course it's natural. Humans and other mammals have what's called a diving reflex where the body will immediately start slowing heart rate and oxygen consumption so you can remain under water longer. Also, the air in your lungs from simply holding your breath is enough to keep you afloat. We mostly land-based creatures evolved to be quite good in the water.

Women do have higher fat levels than men, but I doubt that evolution had much to do with swimming and it's a result of evolving higher fat levels for other reasons and it happens they get an advantage when it comes to swimming because of it.

Of course it's natural. Humans and other mammals have what's called a diving reflex where the body will immediately start slowing heart rate and oxygen consumption so you can remain under water longer. Also, the air in your lungs from simply holding your breath is enough to keep you afloat. Humans evolved to be quite good in the water.

Women do have higher fat levels than men, but I doubt that evolution had much to do with swimming and it's a result of evolving higher fat levels for other reasons and it happens they get an advantage when it comes to swimming because of it.