Personally not a fan at all of the Canon (or Nikon) 8-15mm full frame fish eyes. They basically have a circular image at 8mm that I never ever touch, and then a normal wide fisheye at 15mm, that is similar to the 10mm result on the Tokina. I like the ability to zoom in on the Tokina and there is nothing on Full Frame that can do that, other than with the FCP-1 now
I have been an amateur underwater photographer for a few years. I have a passion for travel and capturing my trips in the best way possible. So when I also started diving, I soon picked up a camera to capture that aspect as well. Started off with a TG5 and when I got comfortable with that, I got a Hugyfot housing for the Nikon D500 with the Tokina fisheye and 40mm macro. I moved to mirrorless for above water and I am considering making this move for underwater in the future so I don't have to bring two bodies along. However, the Tokina 10-17 is rather unique and the only real alternative to it would be the Nauticam FCP-1. I am still holding off this decision for now due to the high cost involved.