Making photos under the water with common camera

You generaly need air in front of the lens but the waterproof housing to protect the rest of the camera will provide this.
The distance scale on the lens and the focal length (angle of view) will be wrong but otherwise everything, including autofocus will work.

edit everything is very blue which might confuses the auto-white balance on digital cameras.

 

Other than keeping the camera water-tight, the camera will work perfectly fine in and out of water.

 

If water gets into the camera, it will likely ruin the film or short out the electronics, or both.

If you conformal coat the camera with something like RTV or PDMS, it will still take photos underwater.

 

In that case the front element of the lens will have almost no optical power.
The lens elements bend light because of the difference in refractive index between air (1) and glass (around 1.5), if you replace air with water (1.33) the difference in refractive index is much less and the bending power of the lens is reduced by 75-80%.

It depends on the design of the lens but generally most of the optical power is in the front surface of the first element (especially in wide angle camera lenses) so I wouldn't expect most lenses to produce an image.
notes - an element is each individual piece of glass, the lens is the whole assembly.

It is possible to design lenses that assume the front element is in water but for underwater cameras it is easier to just put a flat water proof filter in front - being flat it has no optical power anyway - so nothing changes.

 

BTW, I meant to jump in here earlier, but we have had power interruptions. There are "universal" plastic camera bags that are waterproof and have a threaded optical flat on the front. As long as you can actuate your camera's controls through the plastic, you're good to go. Depending on the model of camera (and it's good if its a popular and long-running series of cameras from a popular maker), you can also find custom-fit hard plastic waterproof housings for your camera with sealed external control-buttons.

Google is your friend "waterproof camera housing"
http://www.google.com/search?hl=en&q=waterproof+camera+housings&aq=0&oq=waterproof+camera+housing