I use the Nikon 14-24f/2.8 on the 5DMK III with a Novoflex adaptor, requires Manual Focus, but it's Landscape so works fine.
Works better on the D3x.
I also use the Canon 8-15f/4, mostly @ 15, find it's a pretty good Lens at 15, certainly better than the older 15f/2.8 Prime.
The Canon 16-35f/2.8 I've almost given up on, all the Canon Primes in this Range work much better, 14f/2.8, 24f/1.4, 35f/1.4, and the Zeiss primes in this range work better than the Canon primes, as long as your Ok with Manual Focus.
I have the 15mm and like it a lot especially with the software we have nowadays like dxo that corrects the aberrations and let's you de-fish it all or partially. I was interested in getting the 8-15 but I am not really into circular shots and I didn't think it was supposed to be any better at 15 than the prime? Is it?
I use both primarily for Underwater Imaging, the newer 8-15f/4 is a much better Lens @ 15 than the older 15f/2.8 @ f/4. The usability of the 8-15 is more restricted as you get lower than say 12mm, at this point it's more about Fun than good Images, still , fun is what Photography is supposed to be about.
For Lanscape work I lean towards the Zeiss 15f/2.8 & Zeiss 21 f/2.8 where the Manual Focus isn't an issue.
The Nikon 14-24f/2.8 is better than anything that Canon currently makes in the Range, but this is a subjective viewpoint, although based on My owning both the Canon 16-35f/2.8 & the Nikon 14-24f/2.8, I've used both these lenses extensively in Underwater Imaging and Landscape, the Nikon beats the Canon in every way, unfortunately. I'de love to see Canon bring a 14-24f/2.8 to Market with the sharpness of the 17 & 24 TSE lenses or even as good as the Nikon.
You might want to give the new 16-35 f/4 a try. I am not sure its suited for underwater photography, I think that's an an application like astrophotography where a faster lens is desireable but for general landscape shooting it's great. it doesn't go as wide as some would like but it goes to 35 and that is a plus for me. It's sharp at f/4 and doesn't need to be stopped down unless you want more dof.