The 590 is the better camera for underwater use. The G10 is severely limited in it's ability to accept wide angle lenses and a few more mega pixels and RAW don't make up for that. If macro is the only interest then the G10 is a better choice. This mega pixel and RAW thing is beginning to take on shades of urban legend beyond simple physics and actual print requirements and most cameras today have vastly improved onboard processing. Unless post manipulation is a keen interest, RAW may not be such a big deal. IMO.
If you are getting the camera for underwater use the 590/Ikelite is the better choice. It accepts a range of underwater lenses. Think of the camera not alone but as part of a system. As part of a system the G10 flunks due to the limited options for wet lenses, the restrictive ports, the adequate but hardly wide angle and kind of weird dome on the Ikelite housing for it and the ridiculous and severely limiting square port on the Canon housing for the G10.
The Megapixel Myth
By the time you save up money to get a housing for the 40D none will be available and it will be completely obsolete as a result. If that is your interest, get it now and bypass the 590 and the disappointing G10. Hire a couple of sherpas to tote it for you.
Also interesting, go to this fellow home page and then look at his tech articles:
A $150 versus a $5,000 Camera
Oh, and this RAW crap:
RAW vs JPG
So, what makes the G10 better?
Not u/w but I took this pic when I was about six years old with a camera I got from coupons on a Rice Crispy box, I still think it is my best ever and it was my first:
You know what, G10 my a-----.
N