...I just ordered a 17-40.
The 24mm f1.4 is really quite good. I was very satisfied with the results and this is the lens I plan to use for most of my Bahamas shark shooting...
That's actually the primary reason I got the 24mm, too (for dives where sharks, dolphins, mantas, etc. are the main targets). I went back and looked at the details of photos I took on my previous Shearwater trip, and found that most of shots I liked were either with the 17-40 zoomed in the 20-28mm range, or (surprisingly) with my Sigma 50mm macro lens (which I REALLY like as a general fish lens, more so than for its macro capabilities).
On this upcoming Bahamas trip, since I plan to use a Magic Filter as often as possible (when there's sun), I figured having a sharp and fast lens like the 24mm would be a great option for shooting with just ambient light. The 17-40 is a great lens for FF, too (and most people see little difference between it and the considerably more expensive 16-35mm), so I think you'll be really happy with it as well.