Thanks!
Well all you really need are a camera and a underwater housing. A strobe helps if your diving deeper then 10m or so. There are even a few point and shoots out now that are rated to 10m so you don't even need a housing anymore!
There are housing made for most cameras out there. Everything from a basic point and shoot to the top end DSLRs. The material for housings are usually either acrylic or aluminum depending on brand and build quality.
Hope that answer most of your questions. Feel free to ask some more if you like.
I do warn you though, underwater photography is almost as addictive as reef keeping.
Cheers,
Tom