For shooting sharks, and any other marine life for that matter, there are a number of rules that will greatly enhance the quality of your photos.
Firstly there is the light and colour drop off caused by the water. The density of the water causes the red part of the spectrum to be filtered out of light in the first 5 meters and as your depth increases the colours decrease until you are left with dark blues and eventually black. To counteract this colour drop off, you can either manually white balance your camera with the surrounding area or use strobes to "paint" the colour back into the picture.
The second rule of photography is to get close to the subject then get closer again :). This is because water contains particulate matter that gets between the lens and the subject and generally degrades the quality of the image. The other reason for this advice is that many photographers take the shot with too little of the subject in the frame.
To shoot the photo above I was using a canon 5D with a 17-40mm f4 L lens and a B+W -4 diopter to correct the distortion caused by the dome port.
As the shark was too far a way for the strobes to light it, I manually white balanced the camera using my hand, as there was no reef / white card available. Camera settings: Top shot: 1/125s
f7.1 ISO 250 @ 40mm.
I have included the second shot to illustrate how close these sharks get, Here he was about 2 feet from the front of the dome port and the shark was getting frisky, you can make out a divers legs behind as he struggles to get out of the water into the zodiac :) Sadly the whole encounter only lasted less than a minute and It was the only dive I was able to do as I managed somehow to get a very bad cold and fever for a couple of days, which, sadly, put an end to my diving.