When I think of a sport, I think competition. In a sport you have winners and losers. Diving is as much a sport as hiking, mountain climbing, and white water rafting.
In diving the winners include everybody that participates. (Extra point is having a good story to tell about your dive, whether true or not.

It is my life outside of work because it is peaceful and quiet. It is my meditation time and I very seldom want to come back up. I think through my problems during a dive as being under the water puts everything in a different light.
It is also something that not everyone does and I see sights that not everyone will ever see. To me diving isn't a sport, it is an adventure that makes me what I am. I am only sorry that I didn't take it up fifteen years or so before I did.