Wendy,
Spend enough time reading the forums here and you'll see James and I have some fun, spirited debates
So I must launch another volley!
Viewfinder point:
* I and a friend have used Canons's whopping <$20.00 EPX-15 viewfinder magnifer to enlarge (while making a slightly farther away view) the eyepiece on the original Digital Rebel and will likely do this to use the Rebel XT. Some minor grinding off of a bit of plastic, but a great solution for making the Ikelite SuperEye even better.
* Seeing the edges of your frame super accurately while shooting underwater? I think that's the biggest joke in the digital underwater world! You're moving, the fish is moving, your mask is a large volume model, small volume model, etc. etc. etc.
Look over photos posted here and on any UW photographer's site. Many are WAAAYYYYY too far away and include negative space that degrades a photo's impact. Did a "better viewfinder" help in "framing accurately? Did the photographer really intend to leave that much negative space around their prime subject? I DON'T THINK SO.....Learning to "push in" your camera as close as possible is the way to get good photos. Which leads me to the next point......
* As I've stated on this and other UW photography forums numerous times you should shoot as close to the edges as you can view, knowing most dSLR camera show only 95-96% of the actual frame anyway. Then there is this handy tool on EVERY Photoshop program called a "crop tool" enabling you to tighten up your image during post processing.
One other thing people debate ad naseum is frame rate as in the Rebel XT / 350D 3 FPS verus the 20D's 5 FPS. Tell me one UW photographer who has used this underwater!!! No strobe can keep up! Maybe for some available light shooting with fast moving dolphins, or on the swim step during shark feeds, but otherwise, what horse puckey!!!
For my $$$$$$ having the following features on the Rebel Xt / 350D are the most important to good images. And at a lower cost....
* DIGIC II processor for low noise / dynamic range capability, faster AF and file processing
* ETTL II flash control
* Adjustable AF, metering, White balance and other things "crippled" on the original Digital Rebel.
Can't think of other features, but I'm sure there are many.....Read the Rebel XT / 250D reviews for comparisons between the two cameras and then decide....
For what it's worth, my brother has a Canon ESO 20D with 17-85mm IS, 50mm F1.4, 100-400 IS and the 580EX flash!!! I've tried it, liked it but when it came down to it I bought the Rebel XT and have zero regrets....I may however borrow some of his Canon "parts" as they are 100% compatible with the Rebel XT 
IMHO more time traveling (and actually diving!) plus practicing at home with your camera will make you better photographer.....
What fun!
David Haas