lots to remember - if your around London have a chat with Paul at cameras underwater.
Biggest things to remember are that water acts like a blue filter which absorbs roughly one stop of light for every 2 meters in any direction. So as well as all the other issues this causes everything tends to be very blue, shot in raw so you can adjust WB afterwards
Everything looks bigger too, so it has an affect on min focus distances, you may wish to counter this by adding diopter adjustments to your lens ports
Flash is essential, but equally important to get it of camera, even the clearest sea is full of suspended matter and it will backscatter if you don't and disfuse the flash.
Wideangle lenses are the order of the day, the issues with light travel through water really preclude the use of telephoto lenses
Most importanly you need to keep an eye on your dive still as your highest priority. I know it sounds obvious, but I watched a new camera diver suck his tank dry earlier in the summer, panic at 25m and have a very 'interesting' and fast trip to the surface, followed by an equally fast trip to hospital.
Hugh