I have a d30, 1d, 1d Mark II and 20d. The d30 is being converted to infra-red, the 1d is mostly a backup these days. The Mark II is ostensibly my primary camera, but I find I actually shoot with the 20d more often. It is a terrific camera. Lighter than many other digitals, 8megs, clean images and relatively simple to use.
I haven't used it for underwater work, although it would probably be the SLR I would choose. (For underwater photography, I use a G1 in an Ikelite housing.)
Here is a photo I took yesterday with the 20d (it is actually 4 photos, but don't tell anyone).
A couple of points, while the 20d is a sturdy camera, it is not built to the specs of a true professional camera in terms of being rugged, sealed, etc. That is just something you should know. In any inclement conditions, you would want to make sure you have it well protected.
The only other problem I have found is mine seems to show low batteries very quickly. I think it may be in the optional two battery mount, but it is somewhat annoying. Not a deal breaker, and battery life is still a lot better than with older digitals, but a factor to remember.
At any rate, I would certainly recommend the 20d.
Safe Diving,
Jeff