I'm afraid you might be a bit let down by the quality you get with any sort of "on camera flash." Whether you're shooting with the little built-in flash or something like the SB-600 on your hotshoe, the images are going to come out flat given that your light source is coming from the flash itself. You might consider putting an SB-600 or better yet, a strobe, on a tripod away from the camera and triggering it remotely.
Do checkout
www.strobist.com, and especially look at the "lighting 101" section here:
http://strobist.blogspot.com/2006/03/lighting-101.html
It's a website about off-camera lighting written for amateurs in easy to understand language.
Also, like others said, look at getting a 35 or 50 mm lens that will stop down all the way to f1.8. That will DRAMATICALLY increase the amount of light that enters the lens, allowing you to use a faster shutter speed, which is absolutely essential for freezing action. Of course, it'll result in a much more shallow depth of field. Depth of field is the amount of the image that is in focus... meaning that the bigger the aperture is, the lesser amount of image will be in focus. Look to this link for a good explanation:
http://www.cambridgeincolour.com/tutorials/depth-of-field.htm Using a lens like that will also mean that you can't zoom, but bear in mind that these are the types of lenses the pros typically use, and for good reason. They almost always have better image quality.
By now, you're likely understanding that photography is all about tradeoffs. The trick is learning to use those tradeoffs creatively to your advantage. So what if you get a grainy image... make it black and white and make it EVEN MORE GRAINY in photoshop... the results can be pretty cool. Maybe go ahead and use a SUPER SLOW SHUTTER SPEED and get tons of light and capture the skater in motion blurring past you.
I may be a novice with reefkeeping, but I do happen to know a thing or two about photography. Hope this helps.