After several dives with the fisheye, correcting the distortion seems rarely necessary. Sometimes you like to and it's easy enough.
The fisheye has the additional advantage of being very flare resistant. It is quite wide, though, at about 102 degrees (linearized), I think.
With the fisheye, I mounted the filter in front. I purchased an extra Nikon lens cap, cut the front off, pressed in a 62-67mm stepup ring and used a Singh-Ray FL-B. the corners were very slightly clipped. I think a 62mm Tiffen FL-B would work and not clip the corners. The effected area is very slight. A rear mount would probably work better still but Nikon uses a bayonet mount and I don't know how to fit gels into it.
My main goal of the trip was to learn how to use the strobes and camera in full manual and to play with filters + strobes. I spent a good deal of time looking at RAW histograms since that tells you most about the light entering the lens. For closeup, the green notch of the FL-B was bad although the color tilt itself was not. For light path distances around 6-10 feet, the color balance became quite good (assume strobe power was right). For longer paths, the color balance shifted increasingly toward blue no matter what, but I was not using big strobes. For almost everything we shot the filter was not too strong. Of course, color correction is required, but that is always true when shooting RAW (or anything else, actually). Images straight out of the camera were suprisingly good, but the two of us shooting were still learning and not always getting things right.
I was less pleased with the Sea and Sea housing. The mounting tray is nice but it adds bulk and introduces some tolerance issues that cause the controls to be unreliable. My function knob does not work at all, and the 4 toggle buttons frequently refused to work and would engage themselves. The manual focus control frequently activates itself (ruined a dive because Mike didn't realize it) and the focus area moves around on its own unless you lock it. I intend to sell the housing and switch to Nexus or Aquatica. I believe the control problems could be worked through and I'm sure others will be quite happy with them.
Thanks for the compliment of the hammerhead. On that dive, the focus area was locked at the bottom and it was nearly impossible to focus, then compose since the shutter lever is so sensitive. I blew at least a dozen shots better than that one, including some full frame head shots. The big difference between still and video is that still is so infuriating! The other diver using my rig got an incredible hammerhead shot which I'd like to share but want his permission first.
