Why can't humans swim from birth?
How can we be descended from aquatic apes, and have forgotten how to do that?
Why can't humans WALK from birth? Obviously, we must not be land animals.
Seriously, human infants can swim. I once saw a video of an experiment in which a young infant was put in water, and it instinctively began kicking and paddling with its arms. Apparently, this ability disappears rather quickly in an early child's development, unless it is nurtured. I understand that it disappears entirely after the first six months or so.
Moreover, the Aquatic Ape theory holds that we (obviously) never completed the transition to a purely aquatic species, like whales, or even sea lions.