At the Olympic Games, aquatics means the water sports: swimming in a pool, diving from a board, water polo (the ball game in the pool), artistic swimming (the one that used to be called synchronised swimming, with music and held poses), and the long race in open water. Those are the five. Though actually, outside the Games people use aquatics more loosely for water sports in general, or for the building where you do them — an aquatics centre is a big public pool, often with diving boards and a warm teaching pool as well. The word feels a little official; a child going to the pool would just say swimming. You almost always meet it as aquatics, not an aquatic. Teachers and sports news like it because one word covers the whole water family of events.