Before you can dive into the water with your camera, make sure you have read the directions for use very well. In some instances there is a certain depth that the camera can handle and beyond that it would probably get damaged. Make sure you know about this and make sure you ascertain how far you can go with your camera.
Most waterproof camera housings will come with a rubber 'O' ring, or grommet of some kind. These rings sit in between the main housing, and the opening for you to slide the camera in. It is vital that you regularly inspect this rubber ring for any defects. It might pay to buy one or two extras in advance because it will be very difficult locate one on vacation, let alone from a boat out on the reef where you are diving!
Apart from all this, try to secure some flooding insurance which will vary according to the replacement cost of the camera in question. If you have a camera that is priced at a lowly amount of $200 and it is a basic camera then there might be no need. However if it is an $1800 Canon SLR, then everything changes completely.
Finally, you need to be careful when removing the camera from the waterproof digital camera housing after use. Dry off the housing so that there isn't any visible water that will leak in when you open the seal. A towel will do, but a hairdryer on a low heat setting is best if you have one handy. It would also be good to give the housing a good wash if you have taken it in to a salt water environment. This will keep the unit clean and clear from any salt residue that dries on the surface.