For underwater, we have to make compromises because the available camera, lens, housing, or port might not coordinate well. Before you get locked into a choice of camera or lens, make sure there is a housing and a port that works with them.
Underwater a most useful lens is the Tokina 10-17mm. This is a very wide fisheye lens. A fisheye lens is usually OK for underwater because there are few straight lines, it is very sharp in the center and reasonably sharp in corners stopped down, and it allows you to get very close to the subject for best visibility.
The Tokina is a DX lens, so is less effective with a full frame camera - it will vignette or give a cropped circular image. Only for that reason, a D700 is not a great match for it, while a D300s or D7000 is. There is a fellow in Germany who will mill off the integral lens hood of the 10-17 to give it a slightly wider capability, by the way. It's chancy, because then the lens does not have the physical protection from scratching that the hood offered, but for dedicated use in a housing, this can be useful. Some people say adding the Kenko 1.4x TC allows an almost full frame image on the D700, worth checking out.
The 10-17 is not an AF-S lens, so is poorly suited for Nikon bodies without focus motors like the D3100, etc.
The 10-17 is not threaded and does not allow the use of threaded filters such as a "diopter" (common misnomer for a close-up or magnifying lens) but it does not need it in practice.
It mates well with a dome port that is properly positioned such that its nodal point is equidistant from all points on the surface of the dome. The dome position
must be matched to the lens nodal point for best corner sharpness.
Regarding the size of the dome port, the 10.25 domes are inconveniently large for my use as a traveling diver or for getting into small nooks and crannies to get close to a subject. There are smaller domes with 4-5 inch diameters that work as well for close focus wide angle (CFWA) use with the 10-17.
Recently Zen released a CFWA glass dome for the Ikelite housings using the 10-17 ($900); I suspect other mounts will follow:
Zen Underwater DP-100 100mm Fisheye Dome for Ikelite/Tokina 10-17 [zen.dp100.it] - $899.95 : Reef Photo & Video!, The Underwater Photo Pros
Two upstart custom houses also make CFWA acrylic dome ports for the 10-17 on various housings.
Barry Guimbellot's dome for Nexus M5 and M6 housings, for quite reasonable prices ($360):
Creative Illusions Photography--Port
Another vendor, with an Ikelite modular dome for the 10-17 at reasonable price ($300-350):
5" Modular Dome for the ikelite modular port system
That's a snapshot today, but more choices will emerge in time.
My point is that you have to think through your whole system to make sure all the parts coordinate well. There are good reasons to choose a DX body to match preferred lenses, even though on land the FX bodies and lenses are splendid.
16-35 seems to have the same issue as the 14-24 and for a traveller...
