Rob,
The Canon 17-40L is a great lens and for surface work on a Canon 5D produces stellar results. Underwater any zoom will be good, great, or in between depending on what port and / or diopter to sharpen edges combination you try. Plenty of posts here in the past on using wide zooms and related edge sharpness problems. I vaguely recall many zoom users wiht 8" dome ports afdding like +2 or something to sharpen up the edges....James Wiseman might chime inas I think he's had experience with this.
This is also an oxymoron as many of these zooms focus to less than 1' and theoretically don't need a diopter to "see" the apprent image a dome port creates..So many zoom shooters are just trying to sharpen their corners a bit by adding a diopter.
Using a Sigma 15mm full frame fisheye is what you WANT in a wide lens UW. Allowing you to get close where your strobes will have effect or even for available light to let details be captured. Fisheye lenses can be used UW at certain angles, (not up or down to showcase the cruvature a fisheye lens creates) and to most viewers would just look like a really wide lens. Plus with a fixed prime angle lens you won't have edge sharpness dilemmas as described above.
HooverMD in choosing the Sigma 14mm will have a really wide rectilinear lens, which as long as he doesn't include the sun at sharp angles (siince this lens flares Mucho' as posted on surface and UW forums) he might be happy with it UW. Shot at reasonable apertures (f5.6-11) it is very sharp for a non-manufacturer lens....But the Sigma 15mm Fisheye is still a better buy IMHO.
Finally, Alex Mustard while using the Nikon 12-24mm zoom occassionally (and maybe more often, Alex chime in please...) has shot so many wide angle pictures with the Nikon 10.5mm full frame fisheye lens on Nikon dSLR bodies. Some look "fisheye", but many don't depending on the angle the photo was shot.
I have lately come back to a Sigma 15mm fisheye on ebel XT (and Canon 5D ) and a mid-zoom on my Rebel XT, the Canon's 17-85mm. Plus maybe an occassional macro lens the Sigma 50mm. This simplifies my shooting style allowing me to wring every bit of capability out of a tool versus changing constantly and getting mediocre results due to unfamiliarity with a particular lens.
Hope this helps all of you guy's decision making!
dhaas