Hi there,
I've always used Aperture Priority when using my Canon S95 with my wide angle lens and have always loved it. Keeps things nice and simple for a beginner and even for me having been shooting for 10 years now with compacts.
If you don't have an external flash, then using the camera's manual white balance for reef scenes works really well. Choose an ISO of 100 if you have a wide angle lens, as the lens itself will let in some light. I would choose an aperture of f5.6 for reef scenes, or an aperture of f8 when using a wa lens in shallower water or where there is a lot of light.
With an external strobe, I would choose a lower ISO of 80 or 100 and again a small aperture of f5.6 or f8 depending on the available light and time of day that the photo is taken.
This attached photo was taken with a simple Canon Ixus 980 (called Ixus 990 in the States). I used the camera's macro button which changed the aperture to f8 whilst in forced flash mode and then chose an EV of minus 2. This helped to reduce the sunburst behind the reef scene.
Hope that this helps a bit. At the end of the day, whether you are on Programme Mode, AV, TV or Manual mode it is all about having fun and enjoying the dive. Even in Underwater Mode I've produced some award-winning shots! The important thing is to come back with photos that you like
Maria
PS Personal Tips for your Compact Camera set-up as well as my book especially for Compact Camera Users are available on my website at
http://www.oceanvisions.co.uk