Underwater photographic equipment cannot be watertight unless the O-rings that seal it are flawless. Always carry spare O-rings in your camera case.
O-rings are the most important components of your underwater camera system. If the O-ring is lubricated, it will be elastic enough to shape itself uniformly around the channel. If it is dry, there will be too much friction to allow it to reshape itself uniformly. It will not safeguard against flooding.
Every time you open the camera to change film, examine the O-ring and service if necessary.
Always clean and lubricate after every diving day.
When in doubt about the sealing capability of an O-ring, always discard and replace.
Never use a sharp or pointed object to remove an O-ring! Use the O-ring remover tool.
Don't overgrease! Too much grease attracts debris that will disrupt the seal and cause flooding. Silicone grease is not a sealant; it only protects the O-ring from abrasion. A perfectly greased O-ring looks moist and shiny.