I use the Canon 14mm II rect., 16-35mm II, and 24mm, as well as the Sigma 15mm FE. Surprisingly, I find that I tend to go with the cheapest one (Sigma 15FE) first and most often. I like the 14mm, but underwater you really don't need the rectilinear, and the 15FE gets more in the picture. The 16-35mm obviously has the advantage of being able to zoom, but you've already got the 17-40L (a very good lens in its own right), so I think you'd be disappointed with the relative benefit os getting a 16-35 II versus getting a 15FE (Canon OR Sigma, although the Sigma is cheaper, generally considered just as good, and focuses closer). Besides, if you expect to be close to your subjects (e.g., mantas, sharks at Tiger Beach, wrecks, etc.), you'll probably find that you shoot at the wide end most of the time, and with the primes if a subject is occasionally farther away, you've got 21megapixels, so you can crop half the frame away to digitally "zoom" and still have a good sized hi-res result anyway. So I actually don't use the 16-35mm all that much, unless I have no idea what I'll see on that dive. If you're expecting mostly mid-range subjects, the 24mm is a great lens, but it's not overall as useful as the other three in my experience.