Hello there, I'm not very sporty, so someone may contradict me . . . :-) To me, both 'coach' and 'instructor' are fine. You talk about a football coach - that can be both a professional career, or a part-time voluntary activity. I have found an Internet site which defines instructor as someone who teaches a specific set of skills, while a coach develops the person in a wider sense. But for your situation I would say both would be quite correct.