A good way to get a unique composition and a better looking photograph is to shoot at your subject from underneath it instead of above.

While not always easy to accomplish, try to get below the fish for a far more interesting angle and lighting and shoot upward. Often while shooting downwards, your backdrop comprises of the reef or the sea bed which can clutter the image. If you cannot get underneath the subject, try to at least get eye level with your subject for some interesting results.

Just remember while shooting upwards that in bright sunlight, this can cause your subject to silhouette or fall in the shadow. Ensure you use a strobe and get in close to ensure you capture the subject better