Fogging occurs when moisture in the air inside the housing condenses on the cold lens port — typically when the camera warms up during recording in cold water or cool air.

Prevention (do this before every water session):

1. Anti-fog inserts: Small absorbent strips (often included with the camera or available for a few dollars) placed inside the housing absorb moisture before it can condense. Replace them when saturated.

2. Assemble in a dry environment: Seal the housing indoors in air-conditioned or dehumidified air, not on a humid beach or boat deck.

3. Silica gel packets: Place a small silica gel desiccant packet inside the housing (in a spot where it won't block the lens or buttons). This is the cheapest solution.

4. Anti-fog solution: A drop of commercial anti-fog liquid or diluted baby shampoo wiped on the inside of the lens port creates a film that prevents water droplets from forming.

In a pinch (field fix): Spit on the inside of the lens port, rub it around, and rinse lightly with fresh water. It sounds unpleasant, works surprisingly well, and is what scuba divers have done for decades.