Frankly I would not change a thing. For 20 images per year you have a great setup. Remember, camera does not matter, photographer does. You (and me) just do not shoot enough underwater to be any good. It is all practice.
I used 4MP Canon A80 for years with Canon housing and then changed to Canon A710 for IS. Pictures are not any better. I do not even own any strobs, but play with WB instead. You can see results at my website www.redberryphoto.com
If you insist an a new toy, look into Canon SD880 which has wide lens (28mm equivalent), pretty rare in compacts. Canon's housing available from Adorama. Check http://www.kenrockwell.com/canon/compacts/sd880.htm
Hope I just saved you a pile of money.