Anni:
Im a relatively inexperienced diver and underwater photographer so Im not the one to be giving advice. From what little I know I can recommend the following:
1. Practice using your camera in its housing above water. Especially practice using the macro setting. That way you will be more confident underwater.
2. Without an external flash, everything will appear blue beyond 3 feet. Thus limit most of your shots to within arms length. Setting your camera to macro mode, wide angle, will work best most of the time.
3. Use the antifog on the internal surface of the housing lens port. Dont apply it to the camera lens. It will leave a rainbow 'soap film" on the port and cut down on resolution but it is better than having the port fog and loose everything.
4. Use the highest resolution JPEG setting. Each image will take about 1.5 megs. An advanced user will use the RAW mode and adjust color balance later.
5. Follow the basics of underwater photography. Approach your subject slowly, or better yet, let it come to you. Get on the same level as your subject, subjects photographed from above are less effective. Pause your breathing while photographing so the bubbles dont scare or obscure the subject. Dont hold your breath while rising so you dont damage your lungs.
6. Having perfect buyancy control is a must. For those of us (such as myself)without perfect control, dive slightly overweighted. A finger placed on the sandy bottom while the other hand is holding the camera is an effective way of staying still, and not damaging the reef.
6. A dive light can be a cheap light source, and helps the autofocus in low light conditions.
7. You can fix most problems except blurred photos using Photoshop or similar program after the dive! Even a pro photographer will throw out at least 90% of his photos.
To see what a camera (the SONY, very similar to the Canon) can do without an external flash, and using just the internal flash and dive light, see Ellen Mullers gallery at pbase.
http://www.pbase.com/imagine/sony_dsc_p1_underwater_images