I have been shooting fisheye for over 5 years now and I eagerly awaited the arrival of a Canon fisheye quality lens with that wide of an angle.
After owning and shooting over a thousand images with this lens, I feel I have the time and experience to share some of my thoughts.
I love this lens and it's a perfect compliment to any fishing photographers arsenal. It's incredibly sharp and quick. The f/4 allows for great low light images, when bumping up your ISO. I really like how it's a Zoom. The other fisheye I've owned was a fixed 8mm, and wasn't very versatile. If all I'm shooting is at 8mm, everything begins to look similar. But with this lens you can zoom in to 15mm, which is a great length for hero shots without all of the distortion that you find at 8mm.
I shoot a lot out of a boat and this lens comes in handy in the tight quarters that you find in this environment. It would also be a great lens for underwater usage, although I have not yet used this in this situation. My camera I have used this lens with is the Canon 60D. Even with the cropped sensor there is quite a bit of vignetting at the 8mm focal length.. This goes away at 10mm. All of which is expected with a 180 degree view.
Given the circular nature of the lens, a polarizing filter isn't an option, but it's not needed. The sky pops with this lens and all colors are vibrant.
For fishing photographers who like the affects of a super wide angle lens, this a great addition to your canon arsenal. This lens like all of the Canon L lenses is not cheap. Going price these days is around $1,500 new..
Here are some examples of shots with this lens..