Looks like you could do with practicing your focussing on moving objects as most of the shots are out of focus. I see you kept the shutter speed up in the majority (1/500s) so it's not camera shake or blurryness from their motion which I first thought it might be.
I think you could have done with shooting a little bit wider as we can't actually see anybodies kits in the shots, just the harnesses.
Did you use thd dodge tool on a couple of the shots to try to make the riders pop from the image a little? Because I can see some haloing around them that you might want to try to sort out.
Good attempt at a hard sport to photograph, take a look on google for "kite boarding" and see some of the pros work, then go back and have another go - keep us updated.