Welcome to our obsession! 
I'll tell ya what I've been told, and then I'll tell ya what I'm doing. I'm basically in the same situation as you are. I just bought the Canon 20D and the Aquatica housing and I have been slowly learning about lenses, ports, extension rings, and diopters (oh my!). Others here have recommended that new dSLR photographers start out with macro photography, since its a lot easier to do than wide angle, and then grow into the wide angle photography.
I'm planning on cutting my teeth on macro stuff on an upcoming dive trip in a month and hopefully by the end of that trip, I'll try out some WA stuff.
For macro, I bought the Sigma 50mm macro lens. It lets you do 1:1 with good quality. Other options are the Canon 50 mm macro (only 1:2) and a brand new Canon EF-S 60mm macro (1:1). The EF-S lens is only usable on the Canon 20D and Digital Rebel since is specifically designed for those cameras. There's also a pretty good Canon 100mm macro lens, but the general opinion seems to be its not a newbie's lens. I'm sure others here can comment more on the Canon lenses.
I'm going to use the Sigma 15mm Fisheye for my wide angle selection. This is just a personal preference. There's a Canon EF-S 10-22 zoom lens that a lot of folks are using. I've also heard there are some other WA zooms recently released by othere manufacturers (Sigma and either Tamron or Tokina, I can't remember). You should get plenty of comments on these options as well.
You'll need to get whatever ports will fit your lens choices; there are *many* Ike fans here, so you should get plenty of help there once you choose your lenses.
You can also search the forums. There are plenty of posts regarding lens choices. And if you have a question about a specific lens, ask.
Hope this helps,
Ken