If you obtain a secondhand Nikonos camera from a private seller, or have a Nikonos camera which has not been submerged for some time, taking it underwater without getting it serviced is asking for trouble. The most heavily stressed O-ring seal in the Nikonos V camera (and in the IVa) is that around the film advance lever shaft. It tends to dry out and become abraded, giving rise to a slow but insidious leak into a delicate part of the camera mechanism known as the 'winding stack'.

A full service for a Nikonos V involves complete replacement of non-static O-rings, housing only pressure test, full function test, recalibration of the light metering system, and final pressure test, with fixing of trivial faults and adjustments along the way. The procedure requires camera calibration equipment, a pressure tank, special tools, hole plugs and electrical adapters, and access to the service manual. Individuals who do not possess advanced technical skills and a well-equipped workshop are advised not to attempt it.

Underwater resolving power of Nikonos lenses:

UW Nikkor 15mm f/2.8:

73 lines/mm centre @ f/8.

UW-Nikkor 28mm f/3.5:

72 lines/mm centre @ f/8

W-Nikkor 35mm f/2.5:

62 lines/mm centre @ f/8

An air-corrected wide-angle lens behind a dome port, no matter how expensive, will always give lower edge resolution than a properly water-corrected lens. This is due to the introduction of chromatic aberration at the air-water boundary. If the camera is a digital camera however, or if the film is to be scanned, it is possible to correct for chromatic aberration using software. If software correction is applied, a good air-corrected lens can be turned into a good water-corrected lens, and results similar to those obtained with UW-Nikkor lenses can be achieved.

Source: 'Diver-operated cameras and their marine biological uses', A. Svoboda, in 'Underwater Photography and Television for Scientists', J.D.George, G.I. Lythgoe, and J.N. Lythgoe, 1985, ISBN 0-19-854141-4 (OUP).

If buying secondhand Nikonos equipment, you need some kind of guarantee from the seller. A considerable proportion of the equipment offered for sale by private individuals turns out to be faulty, often in some subtle but insidious way, and spare parts can be hard to obtain. The classic problem of the Nikonos III was that of a cracked top-plate, which was caused by lending the camera to someone who didn't know that you have to remove the lens before you can get the camera open. You'd think that this problem would have been rare, but it was our most common reason for declaring a Nikonos III to be a write-off. The Nikonos II had a cast aluminium top plate, but the author has even seen one of those broken in two by someone who tried to remove it with a hammer and chisel (which goes to prove that there really are people from other planets walking amongst us). Nikonos IVa and V cameras tend to leak into the winding stack, so take off the electronics cover plate and have a good look for corrosion. The TTL flash system of the Nikonos V will cease to work if the sync socket has been flooded (the springs in the TTL contact pins corrode and cease to conduct electricity). It follows that signs of tender loving care are good, but signs of abuse are very bad. The important issue to understand however, is that Nikonos camera systems are like cars; in that they consume parts as a proportion of the operating cost, and occasionally need expensive repairs. Before committing money to a system which is no longer in production, address the problem of how it will be maintained.

Here is the maintenance status of some common and less common items of Nikonos equipment:
Calypsophot (1958-1963):- Collectable.
Calypso-Nikkor / Nikonos (1963 - 1968):- arguably collectable.
Nikonos II (1968-1975):- No spares (o-rings available).
Nikonos III (1975-1979):- No spares (o-rings available). Worth maintaining with reclaimed and substitute parts. Write-off if camera top is cracked.
Nikonos IVa (1979-1984):- No spares except o-rings and parts common to Nik V. Write-off if film-advance return spring is corroded.
Nikonos V (1984 - 2001):- Spares still available in 2006, but at 5 years since discontinuation, this situation may not last much longer.
W-Nikkor 35mm f/2.5 & UW-Nikkor 28mm f/3.5 - last production versions are black anodised. Old versions are plain aluminium and are less valuable. Look out for dented filter rings, scratched optics, stiff controls, and internal flood residues - all of which are very bad news.
UW Nikkor 15mm f/2.8 - there are two versions of this lens. The old version with the back element projecting into the camera throat has no spares, and does not work with the Nikonos IV and V auto exposure systems. The focus scale window is acrylic, and will crack if the lens is cleaned with alcohol.
LW-Nikkor 28mm f/2.8 - This is a 'land waterproof' lens for use in rainforest environments (Vietnam war press photography). It cannot be used underwater, but few were produced and the item is collectable.
SB101:- No spares. Unrepairable if battery compartment is flooded. Housing cracks possible. Best avoided.
SB102:- Spares available. Very badly designed from a service point of view, can be very difficult to repair at the electronic component level. High voltage and low voltage circuit tracks are too close, so tends to die expensively from minor water ingress.
SB103:- Factory recall. Collector's item only. Due to a potential explosion hazard, Nikon recalled all SB103s and replaced them with the SB105. Replaced SB103s were rendered useless by drilling a hole in the case. Some of the drilled SB103s found their way back onto the secondhand market - caveat emptor!
SB104:- Discontinued 2002. Over-complicated design (Nitrogen filled housing, microcontroller-managed circuitry), weird battery pack. Reliable, but probably un-maintainable when Nikon support ceases.
SB105:- Replacement for the SB-103. Discontinued 2002. Well-designed small underwater flash. TTL reset too slow for digital cameras which use pre-flash.
Nikonos RS-AF:- Discontinued. Spares still available in 2003. Ludicrously expensive to refurbish if flooded.