I will echo what others are saying....the 17-40mm in my opinion is horrible behind a dome. I have one, and only used it a few times, as I would get angry at how bad the image looked after I was done filming. So that lead me to use the 15mm fisheye. This lens on the other hand is very sharp, and focuses close. Really the only negative is warped look, which in most cases I don't mind. If you are doing reefs, mantas, sharks open water etc...I think it works fine and the warping isn't a big deal. Again, this is personal preference.
Here is an example of the same camara (canon 5dII) you are using and what the 15mm fisheye looks like.
Now for the Rectilinear, seriously, don't buy the 17-40mm. It is an old lens, and for on a couple hundred more you can have, as Drew pointed out the new 16-35mm with IS. I recently purchased it because I was going to Truk for wreck diving, and knew the fisheye wouldn't look good there. And boy am I glad I got it! It looks really good imo. The corners aren't perfect but 1000% better than the 17-40mm. And just the color you get out of it seems to be better.
Here is an example of the 16-35mm f4L with the Canon 5dII..
I also found the IS to be helpful with the lens...
Hope that helps.