Keep in mind that the Nikonos is a viewfinder camera--no rangefinder, no AF. It is somewhat easier with Scuba to get the focus, since you aren't moving so much. Use somewhat fast film, I'd even suggest print to get some more forgiving exposures. With clear water and the depths you are going to obtain, you should be OK for color and for enough light. Even with that, any dirt in the water will kill the clarity of your images at longer distances.
The 35mm lens is great, but offers somewhat limited DOF close up. The 20mm lens gives much more DOF, but is only usuable underwater (it is also a lot more expensive). As an above water lens, the 35mm is great. I have used it fly-fishing and canoeing with good results.
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(there was a url link here which no longer exists)
A flash probably isn't necessary at the depths you are going to.
If you want the "team cheapskate" advice--get a "Hanimex Amphibian". It is basically the predecessor to the Sea and Sea, and sells for about $30. The images on this page (except the one of the author) were taken with a Hanimex. You do not want to use the onboard flash unless you want to really understand the term "backscatter".
http://www.geocities.com/el_dano.geo/gallery.htm
In general, the III and the V are the best cameras. The III is all manual--no metering, no TTL. Also, no electronics to fry if the camera gets soaked. The V is the final incarnation. The IV is somewhat limited in the manual operation and override, as I recall, but sells for less than the V.
If you can, bring a bucket of fresh water to soak the camera in after the dives. Don't let it dry out salty. If you don't plan to take a ton of pictures, try to plan on not opening the camera until you get back to your clean hotel room. Dirt or sand on an O-ring is death.
Check out the tips on this page:
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Matt
Matt