The Nikkor 18mm lens is one of the more popular housed wide angles to ever be used underwater. Now, with the ever-expanding popularity of Canon cameras, there is nostalgia for that one particular lens, especially since Canon doesn't offer an 18mm prime. However, there is a way to use the 18mm lens on the Canon. Both Cameraquest and Novoflex offer adapters to allow the use of Nikon lenses on Canon. There are also adapters for using Zeiss lenses on Canon, but this test will restrict the comparison between the 18mm Nikkor and the 16-35mm Canon zoom.

I tried to standardize the testing to the best of my ability. I used a Canon EOS1DsMKII for its full frame capability and tripod mounted the camera and shot at 1/250th at F-4. The setup was intended to illustrate a common underwater scenario, rocks and coral near the camera off to the side (the girls holding the exposure slates) and something in the center that is the point of primary focus (the front door of my studio).

18mm Nikkor (142KB, 480 x 320)

16-35 Canon at 16mm (134KB, 480 x 320)

16-35 Canon at 18mm (140KB, 480 x 320)

Identical article at wetpixel.com includes PNG (lossless compression) images

My conclusions:

1. 18mm is much better than 16-35 in the corners.
2. 16-35 is worse in the corners at 16mm than it is a 18mm. At 18mm on both lenses the differences are not so extreme, but the 18mm Nikkor is still clearly the winner.
3. Both lenses have light fall off towards the corners, but the 18mm seems a bit worse than the 16-35. Neither is particularly acceptable in that regard.
4. 16-35 seems sharper in the center. See detail on the handicapped sign. Focus was on the door, manually focused with both lenses to the best of my ability. Type on door isn't too far different with either lens, but I think I still give a slight nod to 16-35. I was surprised the center would be sharper, but gratified to see that the 16-35 glass resolves well at the center.
5. 18mm is a good option for use topside if you already own the lens. It is not good enough to bother buying one, buying an adapter, and then using manual focus and manual aperture. Certainly not worth the hassle underwater I don't think.
6. If you use a 16-35, use F-8 and smaller. Don't blame me when your corners are lousy with the superdome, PVL35, and 16-35 shooting at 5.6. The lens displays definite aberrations at wide apertures. Previous tests I've done suggest that the 17-40 Canon lens is a better performer in this regard.
7. Ideally, the test should have included brackets at different apertures to determine the depth of field that will give adequate corner sharpness. In my practical experience with the 16-35 underwater, that happens around F-8 and smaller.

I am still hopeful that time will bring a better wide angle option for the EOS1DsMKII. Meanwhile, we need to be aware that shooting wide apertures will display lens aberrations ... whether the lens is being used topside or underwater.