I want a flatter stomach, do I have to diet?

As of lately, I’m a very active person. I skateboard 4-8 hours a day, and go swimming very often. I don’t overeat, but I don’t diet. Will I get a flatter stomach without having to go on a strict diet if I do a lot of exercise?
I hardly ever eat chocolate, occasionally I’ll have some popcorn or chips, but not very often. I usually eat 2 meals a day.