Fisheye
lenses
are quite
popular for underwater
use. This is due to the
extremly wide angle which lets you get close to big subjects and the
fact that there are not so many straight lines under water that will
look bent. Therefore you often don't need to correct this
in some software
afterwords. Used correctly you can also use it for divers without
making them look weird. Like the picture to the right. It is my wife
captured at 10mm with a fisheye in the Oslofjord.Above the surface the lens is mostly for special effects. You can correct the fisheye effect in modern software, but I believe you might as well use the normal wides for this purpose. You can read more about these wides here.
The picture to the right shows how the fisheye can work above water. It is easy to snap a self portrait with such a wide lens, but as you can see, the distortion might not be so flattering.
There are two fisheye lenses that are made for the Nikon DSLRs. The other fisheyes are not designed for the digital crop factor, making them useless for the use intended. If you are looking to buy one for underwater use, I would recommend you choose the Tokina. Read below to see why.
Nikkor 10,5mm f2.8 AF-DX G
Being the only choice for a long time for Nikon DSLR's, it has become very popular among UW photographers. It's small, fast, light and covers 180 degrees diagonally. It has a close focus limit of about 14cm which is so close that you are able to get a magnification of 1:5. Like most fisheye lenses it does not have the ability to zoom. It has the ability to use filters for those who use them. Some people find it useful to correct underwater colors. Note that it can only accept gel filters that you cut for the little filters pocket at the back of the lens.
Being the fastest alternative for the widest lenses it might be useful above water. When you correct the image distortion you can get acceptable image quality. The alternatives with normal wides is a full stop slower than this one.
Tokina 10-17mm f3.5-4.5 (AT-X 107 DX)
This lens became available early 2007 and immediately caught my attention. It let's you zoom from a 180 degree coverage to 100 degrees. This makes it very versatile. Used at 17mm the fisheye curves are less visible. Used under water it is hard to see the fisheye effect at all.
Looking at this picture that is taken at 10mm with the Tokina lens it is hard to know that there is any fisheye distortion. The same applies to the more classical wreck shots with the silhouette of a diver around the wreck. You can find more examples here.
Having the same close focus limit as the Nikkor combined with zooming to 17mm, give you an astonishing reproduction rate of 1:2.5. Beware that to be able to use the zoom under water, most housings requires zoom gear that is specific to both the lens and underwater housing.
The lens is somewhat slower than the Nikkor lens. They both focus very fast, and the small loss of speed is not critical in any sense. The Tokina costs a little less than the Nikkor and is definately the best option for wide angle photography under water. Be aware that it can't use filters. If you are into using filters you should go for the Nikon lens.
If you want to read how others reviewed the lens you can have a look at Popular Photography, who have tested the Pentax lens, which I believe is the same optical design, Photozone.de who reviewed the Tokina lens on a D200 or Ken Rockwell's site with a more practical approach.