Hi Moonlight:
I can only give you advice from my own experience as a diver in California, and you can take what you will.
I started with a background in amateur photography... Took a couple of photography classes in college and owned a couple of different SLRs before I decided I wanted to shoot underwater in 1995. I first wanted to see if it was something I even wanted to do, so I went with the lowest cost of entry possible, the Ikelite Aquashot housing for the $10 disposable film cameras. I shot with this for a year (about 60 dives?) before the photography became an obsession (like Woody said!) and I needed something better. Moved up to a Sea and Sea MX-10 35mm point and shoot and its respective lenses and shot with this for a few years. Around 2000 I reached the limits of what it could do for me and there was a new digital revolution happening. I shelled out the big bucks for the newest best thing at the time, which was a 4MP digital Olympus 4000 with a Light and Motion housing and dual Ikelite DS-50 strobes. This ran a couple of thousand bucks. While my images have gotten pretty good as I've learned better underwater composition and technique and improved my skills overall, I've since just recently upgraded via eBay to a new used camera. The camera? A vintage 2003 5MP digital Olympus 5050 with Light and Motion housing. I think this will tide me over for now, and since I'm not going pro, I probably don't need an SLR at the moment. I could certainly put some of the features of an expensive SLR to good use, but I'm seeing some really great improvements in technology that are bound to hit the UW market this year so I'm sitting tight.
Lessons I've learned?
- No crime in starting off cheap to see if you like it and to hone some skills. You can always give away or sell your starter gear on eBay.
- It takes time to develop skills and get good images.
- In my progression I would reach plateaus as to how much I could do with the equipment I had, so I have no regrets over the path I chose. The equipment I started with was simple and as my skills grew, the equipment became progressively more complex but also more powerful.
- Patience is a virtue! Takes time and practice to develop your own skills and style, but the results can be really really rewarding!
Hope this helps!
For me personally, 5MP is just fine, the main trouble being with lower resolution is that you can't enter some of the photo contests. A little disappointing but for me it's enough to just adorn my walls and coffeetable. Also, the other trouble with buying discontinued equipment like this 2003 rig I bought is that parts are hard to find! e.g., Hard to find the wetmate lenses, strobe bulkheads, special sync cords, etc., for this system since it's been discontinued for 3 years. For me though, no biggie. What I have will serve me well until this next generation of digicams come out, and then in another year I'm sure some of that will turn up on the used market. 
