Seriously take a look at the rental rates where you are going to dive. If you are on a live-aboard, they often will load the camera for you, hand it to you when you get in the water, and take it from you when you get to the surface.
This may sound like laziness, but it keeps you from flooding a camera. UW photo equipment takes a serious maintanence schedule (keeping O-rings clean, making sure everything is sealed correctly). My brother and I both had cameras flood on us a couple of days into a week long trip to Chuuk. Reason? We were using the wrong kind of O-ring grease on our strobe connections. Worked great on the camera, didn't work on the strobe.
Flood a camera, and you are looking at a few hundred dollars in repair. Flood a cheap lens (35mm) and you are basically looking at junking the $100 lens. Flood a good lens (Nikonos 15mm) and you are looking at serious money.
The 35mm lens on the Nikonos is good. The optics are actually really good. However, underwater, everything is guess-focus and fairly close. With the scatter in the water and the power of the strobes, you are doing most of your work in <10'. This means that a wide angle lens is very important. When I went to a 20mm lens, the number of keeper shots went up significantly. Why? Depth of focus. A 15mm lens (~$1,500, as I recall when I was buying, cheaper now) would probably help a lot more.
Why bring all this up? Renting gives you access to wide angle lenses cheaper than buying. I say all this, but I own my Nikonos. They are pretty cheap now (NikV in bargain at KEH for $199).
If you decide to purchase, check out the notes on the subaquatic website. He is really good about teaching maintanence. Also tricks like, "If your camera floods but the lens looks OK, point the lens up to keep the air bubble in the lens" or "if you get a flood, try to keep the water out of the viewfinder." It all seems really simple, but if you try to think of it underwater, your camera will probably suffer.
Also, if you do purchase a camera, consider dive insurance. There are groups that will insure your gear. For a bit more, you can add your camera. I forget the name of the company, but they repaired both my camera and my brother's after the Chuuk trip.
If you decide to go Nikonos, get the Nikonos V or the III. The III is the last of the all manual (no electronics) cameras. The IV had some odd auto-exposure mode that wasn't easy to get around.
The 15mm lens is now about $600 in bargain condition at KEH. The 20mm is about $400.
Here are my recollections of the lenses:
1) 85mm Lens--good above water and below. I have never seen anyone use one.
2) 35mm lens--good above water and below. because of the flat front port, this is more like a 50mm lens underwater. Cheap enough that you should have on in your kit. Good for macro (with tubes) and above water.
3) 28mm--water imersion optics. I.e. only underwater use. Supposed to be a very good lens. Also can be used with tubes for macro. Probably better for UW use than the 35mm because of the wider angle and the water-imersion optics.
4) 20mm--as noted above, much better for scenics underwater. UW only lens. Needs viewfinder.
5) 15mm lens--I really wish I had one. UW only. Big glass dome (keep clear of scratches!). Be careful of old non-Nikon versions. Some of these project so far into the camera body as to cover the TTL sensor. Needs viewfinder.