Underwater photography is the act of capturing images while underwater.
Underwater photography can be a fun a rewarding activity. There are underwater cameras that do an excellent job in capturing the images of life underwater. There are disposable cameras for diving in shallow water and there are sturdier models for deeper diving. The camera itself is kept in a hard, plastic, waterproof case. Basically the underwater camera is reduced to appoint and shoot model as the case prohibits any real ability to utilize all of a cameras functions. None the less, there are cameras that are able to perform well and give clear precise images.
Underwater digital photography is possible as well as traditional film photography. The only problem with deeper diving and photography is the lack of light. A flash must be used, and when swimming in water the photographer swims through the particles in the water, and the use of the flash is extremely limited. The particles act as a barrier between the flash burst and the subject and result in a clouding of the frame called "backscatter". This problem can be remedied by using a flashlight and pointing it at the subject or by using a flash held by an arm away from the camera and pointing at the subject.
Underwater photographers can purchase underwater cameras or under water camera housing.
Nikon has housings for its digital D2 model called the Nexus. This housing keeps the camera airtight and dry while underwater. It has almost full functionality with respect to photographic controls.
Subal has created aluminum housings for the professional grade canon EOS-D1. This housing allows the canon full functionality while keeping it safe from water damage.
There are many resources on the internet for underwater equipment. Underwater photography is expensive but it is well worth the price paid for the enjoyment.