One of the problems with underwater digital photography is fogging on the inside of the lens port. I believe this problem is especially severe with the plastic housings, since the best heat conductor from inside to out is the fluorite glass port.
Any residual moisture in the camera vaporizes as the camera heats up during use. To minimize this problem one needs to both minimize the heating, and minimize the available moisture. Minimizing heating is accomplished by reducing the power dissipation in the camera; this also produces the side benefit of increasing battery life.
To reduce power dissipation:
If you use external strobes as slaves to the internal camera strobe, reduce the output power of your strobe to the lowest level the camera will allow.
turn off the LCD display when not in use
use quality high power batteries (more on this in another post)
minimize the battery contact resistance (more on this in another post)
To minimize available moisture I use silica gel desiccant packs inside the housing. I have also found that “baking out” the camera for 24-36 hours before I put it in the housing is a great help as well.To bake out my cameras I open the cameras up as much as possible, and place then under a 60 Watt lamp. This heats up the cameras enough to help rid them of moisture, but not enough to cause any damage. If done properly the camera should be very warm to your touch. You must ensure the cameras do not heat up above the specified storage temperature. (140 °F. for my Olympus D-40s)