I'm so overwhelmed Canon G9, 650 or 720?

We currently have a Canon with an underwater housing that we were happy with, but it died last week. I went to a couple of camera stores and these were the 3 recommended models that my husband & I liked. They all look good, but I'm such an amateur I don't understand a lot of the information provided. I mostly take pictures of cards that I make, my son and dogs (indoor & outdoor, distance & close-ups) and flowers/bugs. Having said that, what is making my decision tough is that when my husband & I go on holidays we take pictures when we go scuba diving. As I said I�m an amateur and don�t understand enough about white balance, ISO, flashes etc and the camera stores have such inexperienced staff they aren�t much help at all with my questions. I�d like my underwater pictures to be nicer and am willing to learn, but I need the proper camera to learn with. From what I�ve read, I�ll have to buy a separate flash to help in the deeper/darker areas. It looks like the G9 is the only one I can do that with (of the 3 noted above). But that means a camera, housing, external flash, camera weights etc. If we decide that we need a less expensive camera, I can't really tell what the main difference(s) is/are between the 650 and 720??? Is it mainly the 10MP/12MP difference and the 650 only uses Vista? I like having a larger megapixel as I crop pictures where the object is father away and still end up with a crisp picture at the end. I don�t know of 12 is overkill though? I have been reading a lot of reviews to help me decide which camera to get. Or have I totally missed the boat and omitted a better camera? I came across this site and thought I'd throw a question out to anyone willing to help out a newbie. Thanks ever so much!

Just a few thoughts for you,
1) It is hard to go wrong with the G9. It's rugged and versatile. Plus the waterproof housing is available.
2) As far as being an amateur -- All three cameras have Auto settings which makes them function as Point and Shoot cameras.
3) All three also have manual control options so you can begin to learn more about photography as you have time and interest.
4) G9 uses proprietary Lithium-ion battery which is more expensive than NiMH AA's for the other two cameras.
5) G9 has faster flash recharge times
6) I don't like the tripod connection location on the A720. It is way over to the side instead of the center.
7) 7MP is enough to make an 8 X 10 inch print so if you have 12MP, you can crop significantly and still print 8 X 10. MP are useful for being able to crop and make large prints. 12 is nice for the very reason you stated.
8) Higher MP number has the downside of reducing sensitivity making low light conditions a problem requiring higher ISO to compensate.
9) What model Canon did you "currently" have? Anything you particularly liked or disliked about it?
10) Do you have a maximum budget set?
11) All of these cameras can use the slave type flash which triggers off the main built in flash of the camera. The advantage of the hot shoe on the G9 is the ability to automatically adjust the flash based on focus distance as well as controlling the flash manually.
12) I don't know what you mean when you say the 650 only uses Vista. You can always use a card reader and transfer images via USB ports.
13) IMHO, all three cameras are proportionally as good as their cost. So you really get what you pay for. I have the G9 and really enjoy it.