The IV-A and the V are both really good cameras, but if you plan to use it underwater you might want to get the V because finding someone to do the service on the IV-A is getting harder and harder.
Remember service on one of these cameras is going to run you about $150 on top of the purchase price. I just had both of mine serviced and pressure tested for the season and it ran me over $400 between service, accessory service, and spare parts that I needed for the season, that was for the pair. I really love my Nikonos cameras and I get a lot of use out of them, but I am a diver, I take them deep, and I am also willing to pay for the service, and do the between service cleaning. Yes it is physically demanding and your body aches when you are done cleaning the camera after salt water use.
Kodak and Fuji make nice disposable underwater cameras for about $15 test your ideas with one of these before you get into a Nikonos system, while they are great cameras I would not recommend them for someone that is just going to take a couple of hundred photos in shallow water. They also make budget 35mm cameras with flash in waterproof cases that can be reloaded at reasonable prices.