Specification sells technical equipment - but just like cars, cameras can be over specified depending on the job you want to do with them. Yes, the latest item will probably have advantages but whether these are advantages to you depends on what you want to do with the end product - in this case a photographic image. The internet is as Alex points out, useful for looking at images from the equipment you are interested in. But its also full of hype, myths and because of what it is, the latest, greatest innovation!
I've never used one but have friends who use 20D cameras - its a good camera and well worth considering housing. Underwater photographers have to think of a camera body and housing as one item - so your initial choice is important because it will probably key you into both camera manufacturer and housing manufacturer. Traditionally Nikon have dominated the underwater camera world but this is changing as the important requirements for film equipment are no longer as important for digital.
If you have a 20D then I'd suggest that you see what housings are available for it and then cost up a housed system for any different cameras you might consider - housing the 20D will probably be you cheapest option and the 60efs macro is a reasonably priced and excellent lens.