There are a handful of cameras available that will go underwater without a housing, but they are generally limited to quite shallow depths. I think Olympus has one that'll do 30 feet, otherwise every camera you look at will need a housing. The housings generally come with a lanyard to put around your wrist and most have weights either with the housing or available to make them neutrally buoyant. You can hook them to your BC with various attachments, but my guess is most divers just carry theirs.
Many of the basic point and shoots from Canon, Olympus, Nikon, Sony and Casio have available housings, but you'll want to make sure first before buying the camera. If you already have a digital camera, post what make and model it is and someone here will probably know if there is a housing available. Most dive shops have access to cameras that are set up for scuba, but many are basically point and shoot cameras with housings which oftentimes are less capable than many combos you could put together yourself.
