The first rule of underwater photography is to make sure your camera is safe and dry. The best way to do that is to check for leaks prior to entering the water using a vacuum valve and pump.
When used properly, a vacuum pump evacuates the air from the housing after it is sealed. If air re-enters the system after evacuating it, it's an indication that there's a leak somewhere in the system. We first use a vacuum pump to remove the air, then check again at a later time to see that the air pressure inside of the housing has not changed.
John Brigham, VP of Ikelite and head of product development, goes through what to do and what not to do when using a Vacuum Pump with Gauge to check for leaks on your housing.