Hi, Basil!
I started with a throw away camera that could go down to 25 ft. (I think) and was disatisfied with the deep blue, poorly defined shots that I got. The cost, as I remember, was around $10.00
Then I progressed to a simple point-and-shot Minolta Weathermatic (This was sometime ago!) and the pictures down to about 25 ft., where there was sufficient ambient light on a sunny day, were okay, but just okay. The cost, I think, was around $75.00???
Then, after doing a lot of reading about u/w photography, I decided to get a "real" u/w camera. The process I went through narrowed it down to two cameras:
(1) Nikonos V
(2) Sea&Sea Motor Marine II
I chose the Nikonos because of my perceived notion of superior optics and more flexibility in bracketing shots by using the ISO setting. I have been more than happy with the Nikonos. Both cameras, though, are excellent choices, in my opinion. I also note that with both cameras one must add stobes and the attendent attaching gear to the list. Also, there are additional lenses and other jimcracks that you will want to consider.
The cost for the Nikonos was ca. $600.00 for the camera with the 35mm lens (The lens is nearly useless without a macro kit of some sort.). The strobe and "tray" assembly cost ca. $600.00.
Then I added a 20mm Nikkor lens for ca. $350.00. There were a few other gadgets, too, but the bottom line was in the neighborhood of $2,100 just to get "started".
Later, I added more lenses and macro equipment and was delighted with the results, but only after about a year of really learning how to use the camera--judging distances, setting strobes and f/stops, u/w composition, and, VERY IMPORTANT, learning a lot about fish behavior!
I have since added a Nikon F100 + housing + more powerful strobes to my list of toys, but I still take the Nikonos along and use it joyfully.
My point in all this is to say in more words what Dee said so economically: It depends on what you want--and what you can afford. However, my advice: start out simply and relatively cheaply until you know that you really have the "bug"!
Scorpionfish