Boy, this is a question a lot of people will be asking themselves. I too have been looking at both cameras and they have a lot to offer. Each has some advantages. But me too, these are my thoughts, I'm not trying to start anything either.
The Canon xTi is smaller, lighter by 1/2 pound, has the digic sensor that many people like, has 3 programable buttons, that I think will be very useful underwater, a very bright and easy to read screen.
The Nikon D80 has a huge bright viewfinder and has most of the practical features of the advanced D200. Probably a better build quality. It is heavier and larger, maybe a bit large for my hands. It has a cool wireless strobe controller (not for UW use!), one programable button, programable menus, very easy to view review feature and more. Some have said the buttons on it are scattered, and I would tend to agree. Probably will be better supported by more housing manufacturers, but both will be popular.
Looking around, while there are many lenses for both, there are many more choices for the Nikon, and the 3rd party ones tend to be a bit cheaper in that mount, IMHO.
You need to go into a quality camera store and ask some questions and play with both. Go home, think about it, and go back again and play some more. Then buy from that source, even if it's a few bucks more, as you will have lots of questions and need the support. don't rip off their time explaining the cameras and helping you and then go buy mail-order, that's not fair.
It's a very important decision, as it's just the beginning of your system, and you will be spending much more on lenses (research them too), housings, ports and more.
Jack