There are several available in a point and shoot format. Some brands are SeaLife, Intova, Visiquest and Olympus. There are also waterproof cameras, but are good only for limited depths.
There is. But there is also casing for regular cameras. However, there aren't casing options for all the cameras. Only some digital cameras have a casing that is compatible with them.
Yes, single use to cases for point and shoot cameras to slrs. The biggest challenge with underwater photography is getting enough light. That can get expensive.
Yes there is.sealife is having variety of underwater cameras.
Like Sealife Underwater Digital Camera (DC600)which is 6 megapixel then
sealife dc1200 12 megapixel underwater camera.