Swimming
Swimming describes a large variety of aquatic activities: competitive swimming, recreactional swimming, aquatic exercise, water sports (such as water polo, synchronized swimming, underwater hockey), or keeping afloat in order to survive.
It is hard to say if swimming is an instinct; human beings develop for nine months while living in amniotic fluid and while we are babies, the "swimming movement" happens by reflex when we are in a non-static freeform environment (such as being held up in the air, or when being bathed). This motion usually starts two weeks and lasts around five months post-birth.
The athletic merits of swimming are great; the mass of water provides an element of resistence with every stroke, providing a cardiovascular workout.