Marco,
here are some tips on floods--
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This is good because it includes an indication of when a camera needs a CLA--
"Your best indicator of need for annual service is the equipment itself; does your Nikonos III or V have a slow return on the film advance?"
If you can test the camera yourself--i.e. you aren't mail ordering--bring an ice-chest with you. Fill it with water and gently dump the camera in. Check for bubbles.
As to the causes of floods, Dan lists "lack of CLA" first and "maintanence" as second. I would reverse Dan Blodget's list. Probably the #1 cause of floods is maintanence.
Note that many (many) Nikonos cameras have a CLA much more rarely than they annual suggestion. I know of one pro who basically doesn't do it--or does it more like every 5 years.
If you are using the camera in salt water, maintanence is key. You must make sure the camera is cleaned every day. If you are doing multiple dives in a day, keep the camera wet (in a bucket) or at least damp (wrap in a wet towel) between dives. You do not want salt crystals forming on sealing-surfaces.
When you change film, be manic about keeping the O-rings clean.
At night, soak the camera to leach out the salt. Clean the seals you can get to. Use lint-free cloths and q-tips (I use cleanroom quality stuff). If not, be sure you look for those fibers the cloth or q-tip can leave behind.
As to doing a CLA on a recent purchase or annually--currently, an annual CLA is about $140. A used camera is about $200-250 (eBay). I have to imagine that Dan isn't getting as many Nikonos cameras in as in the past. At some point, you are better off risking the camera another year or two than doing a CLA. Of course, this doesn't take into account losing the film roll on a dive and the camera for a whole trip.
Here is my experience with my camera--
I purchased a used NikV a long while back. It was 5-10 years old and had never seen a CLA (I know this because I was there when Dan Blodget took it apart and it still had the factory grease). It had a bit of corrosion on the wind mechanism--there was a tiny leak in that O-ring. So, the CLA was a good thing. The wind mechanism was going to break in the next year or so.
I later flooded a camera by using the wrong O-ring grease. This was a bad-maintanance issue on my part. This left me without a camera on a week long trip--possibly my last to Chuuk.
As you get into more wide angle stuff, it is the lens, not the camera, that is the major risk in a flood. A 35mm lens might be $50, but a 20mm is more like $400. A 15mm is more like $1,000.
By the way, the third cause of flooding is jumping into the water with your camera--be gentle! The shock of hitting the water can bump the seals and cause a leak.
If you are worried about a used camera, check Backscatter in Monterey
http://www.backscatter.com
they have a NikV FOR $350--Ii was probably serviced by Subaquatic. Add a 35mm lens for $50 and you have a start. Or, the NikIV is even cheaper. ($300). With the IV, you lose some manual capability.
Or, you can go eBay for $200-250 and add a CLA (and possible repairs).
Start asking why you want the camera. If you want a camera for above water (bad weather, near ocean, etc.), there are only 2 lenses: 35mm and 80mm. I have never seen the 80mm except in a glass case in a camera shop. Underwater, add a factor of 1.5 to the focal length. The 28mm, 20mm and 15mm are only for underwater use. Because of this, they are likely better corrected.
Keep in mind that the Nikonos is a viewfinder camera--not a rangefinder. You guess at distance and set the focus. Because of this, wide angle is your friend. Wide angle buys you more DOF. The number of keepers goes up dramatically when you go from a 35mm to a 20mm lens, just based on focus.
Matt