Well. as usual Rooster has the best crow when it comes to U/W photography! Very sound advice!
I have several Niks in the collection, but only use the Nik V. I also have an F100 with a Subal housing. I use both and am addicted to the autofocus feature of the housed camera. HOWEVER, when I want to travel light above and below the water, the Nik V is the winner by far! Even with dual strobes it is lighter and less bulky than my housed critter. So, when I do that rocky shore dive or am lazy and do not want to carry a lot of bulk and weight to the boat, the Nik V is my sidekick!
That said, I have to admit that U/W photography is not a cheap sport! If you get the Nik V, start with the 20mm or 28mm lens: the 35mm that is usually sold with the basic camera is, in my opinion, only good for macro photography with the appropriate gear. I suspect that you will find that you will want to have the 20mm, the 35mm with macro gear, and the 15mm eventually. However, you can start with one strobe and one lens! Likely you will soon want dual strobes. I like Ultralite trays, etc. with Sea & Sea strobe(s), but there is a plethora of good gear out there. As I wrote earlier, this is not a sport for the miser! For items that do not risk leaks (i.e., not lenses and cameras), you can sometimes pick up some bargains on eBay (listed under "Nikonos" in the Photography and Optics section).
I am certain that I have offended some S&S, etc. users, but you asked about Nik V and Nik IV. If you want to consider S&S, etc., there are some decent options in that group, too.
My final word (I promise) is that a Nik V is a good place to start if you have no experience in U/W photography. The "rules" for terrestrial photography and U/W photography are quite different and the learning curve can be steep if you work at it. Jim Church's book on Nikonos systems will give you some valuable pointers.
The Fish That Only Its Mother Could Love, Scorpionfish