First find a reef scene (Pro’s call this Negative Space) that you like, which could be a beautiful collection of corals and sponges. Reef subjects like this (stationary but interesting and colorful) are excellent for testing and learning about your camera settings and capabilities. If you are shooting without flash then you will need the sun behind you. If you have a flash then have the sun high up in the photo and use the flash to illuminate the reef. In situations like this to get the blue of the sea in the background you will need to get down low and aim at an angle upwards. Very often this shot lends itself to a vertical image. This naturally puts the sun high up in the photo and creates more interest. To make sure the sun’s light does not overpower the photo use a higher Shutter Speed.
You may find that, after awhile, having tried a few different settings in this location, fish life will join the scene and provide a more complete image, worthy of winning a competition! Remember focus the camera on the focal point of the scene and holding down the button compose the photo. Check the results until you are happy. To give the reef scene depth having a passing diver in the distance can help. If you have been doing this for sometime, check your gages and buddy!