Congratulations on getting qualified for SCUBA - well, when you complete your course. This will open up a whole new world for you.
My first piece of advice is to get comfortable diving before you start taking a camera with you. Even when you have completed your course, there's still an awful lot to get your head around and having to worry about a camera and taking photos could be very dangerous. The number of dives you'll need to get comfortable will depend on you, but I'm guessing anywhere from 10 to 50 dives may be necessary. Basically, you want your diving to be second nature and not have to think about stuff too much. Additionally, buoyancy control is very important.
While you are getting your SCUBA experience, you can start thinking about underwater photography.
With SLRs you need to consider beyond just the housing. As SLRs have interchangeable lenses, the housings have interchangeable lens ports. You need to use the right port for the lens. There are also two types of ports, dome and flat. Typically, you'd use a flat port for macro or longer lenses and a dome port for wide-angle. A flat port narrows the field of view whereas a dome port pretty much preserves the field of view of the lens.
As Greg says, you also need to take your own light with you. There's a wide range of underwater strobes available and ideally you want to get ones to match the camera/housing. Note that it is the different rates of attenuation of different wavelengths in water that are the main reason for using strobes. Red light disappears very quickly and even at 5m, available light shots will appear very blue. There are filters you can use that help, but these decrease the light even more and mean you either have to go with slow shutter speeds, wide open apertures and/or high ISO.
Generally, you should stick to wide-angle to short telephoto lenses. Due to light loss and also particulate matter in the water, you want to get as close to the subject as possible and eliminate as much water between you and the subject as possible.
Ikelite have been around for over 30 years and have a pretty good name. Their housings are generally inexpensive compared to some other brands and one of the good thins is they are clear so it is easier to see if there are problems. As housings are specific to make and
model of camera, you have to think about how much you want to spend on the housing knowing you'll need to replace it if you upgrade the camera.
I have been using Ikelite gear for around 6 years and have been happy with them. I have had some recent problems with my D300 housing but Ikelite were helpful and even replaced my 18 month old housing with a new one in order to resolve the problems. As I have an Ikelite housing, I also use Ikelite ports and Ikelite strobes so everything is compatible. Using the Ikelite strobes with the Ikelite housing means I get iTTL exposure, which makes things a lot easier.
I mostly take macro shots and use my 60mm macro. Some people use 100-105mm lenses for macro. I never have myself but often find that (at least around Sydney) even the 60mm results in me being too far away. I also use a 18-55mm lens (for general UW photography) and a 12-24mm lens for large subjects (like sharks, although it can also work with smaller subjects like weedy seadragons).
You can see my gallery (linked below) as well as my
Dive Log which has pictures from dives.
A very good resource is
Wetpixel.
