I have used a SLR system since February 2007. At first a bit frustrating, since my standard lens 14-45mm wouldn't focus with my domeport. After a lot of cursing underwater on the Mozambique and second Bali trip, I bought a 14-54 mm lens and an 8" Dome port. I later found out, that with the 14-45mm and dome, you need a diopter in order to focus. When I have saved up money, I have expanded the system and listed below is my current set-up.
Camera: Olympus E330
Housing: Ikelite
Flash: 2 Ikelite DS125
Ports: 2 flatports for macro and an 8" dome for wideangle
Lenses: Olympus 14-54mm
Olympus 8mm Fisheye
Olympus 35mm Macro
Olympus 50mm Macro
Olympus 70-300mm tele (for above water)
Sigma 105mm Macro
Wetlens Woody's Nexus wetlens
After having read a lot of books on underwaterphotography, and a bit of trial and error, I now only take photo's in RAW. I tend to use manual on both aperture and shutterspeed. If not I now would coose shutter priority.
In my experience all the technical stuff is quite overwhelming in the beginning, but when you get the hang of that, I really think that composition is the big challenge. I want to make photo's that other people would find interesting to look at. Hopefully I have succeded (at least on some of the images).
If you are interested in underwater photography, I highly recommend to take a lot of photo's above water and get 100% accustomed to your camera. In the process you would learn a lot about composition. Join an internet forum to get feedback and take criticism with a positive attitude. Then take an underwater photography course. It is definately worth it, if you find the right guy/girl. I learned a lot on a daycourse with Jeff Mullins in Tulamben Bali. I only had a point and shoot and didn't really like the results. But boy, Jeff could get something really good out of that small system.
I hope that you like my photo's. If you do, you will know that you can get there quite easy.
Enjoy.
Manta Ray, Manta Point Nusa Penida April 2009. 8mm Fisheye F7,10 1/100