Use of light is so important for taking good underwater photos. If your housed camera has a flash then try to use it even though it will have a limited effect on your image. It can bring out the natural colours in the reef and subjects. The limit of your small builtin flash will only travel so far and will not be much use on a wide angle reef shot, but you can use it for close-up photography reasonably well.
It will only illuminate subjects that are between 1-3 feet away, depending on the camera model. The flash coverage is also restricted because it will mainly cover one side of your camera, hence it will illuminate that side of the image better than the other, especially if your have a zoom function and the housing protrudes out of the front a few inches. This can create a shadow in the photo.
The down side of using a flash is the light from the flash could catch and highlight any tiny particles in the water that are between the camera and the subject. These will show up in the photo as tiny white/grey dots known as ‘backscatter’. Hence the need to get closer to your subject, less water in between means less chance of backscatter. It also highlights the need for a wide angle lens so you can get closer to your subject.
For best results I would advise you to buy an external strobe and I recommend the Inon D-2000, which I sell here.
Note: You will need to set your camera on Auto White Balance when using a flash!