I use the Fuji f810 camera for a number of reasons - its size, speed of focusing, macro capabilities and ease of use. It also has a full range of manual settings unlike many digicams which can be important underwater. This has now been discontinued and replaced with the E900.
It is housed in the generic fuji housing though I'd dearly love a shiny ikelite housing
For macro I use an inon wetmate macro lens and for wide-angle I use the inon fisheye wetmate. Wideangle is important underwater as you want to get closer to your subject than you might on land to reduce the amount of light swallowing water and picture quality reducing suspended plankton between you and your subject. Using wetmates allows me to switch between wideangle and macro underwater (something I could not do if I was using a dslr) and hence I'm prepared for anything from nudibranch to whale shark.
Light is lacking underwater so powerful flashguns are needed- I use a pair of inon strobes to get as much light as I need on my subjects. Finally you need to be able to position these strobes at funny angles to avoid light reflecting off sediment in the water and ruinning your picture so you need arms for the strobes and I use the ultralight system.
Put all that together and you end up with quite a rig to push around! When I get chance I'll take a picture of the whole thing to share with you all - in the meantime here's a shipwreck using all of the above (and importantly the fisheye lens):
[Edit: Welcome to anyone following the link from Firefly Forest. I should also mention that if anyone has questions about underwater photography I'll try to answer them, and that far more info is available from digitaldiver.net - the friendliest dive forum on the web.]