Along with closing the housing and camera up in the AC, also refrain from using the camera out of the housing in the water between dives (snorkeling etc.). Everything seems nice and dry until you go out into the heat on the way to a dive and hit the drink and everything is fogged up. A silica packet won't help then either. If you have no AC and it's warm and humid use the silica packets. Heat them up to drive out moisture and then put in housing and close it up as long as possible before diving. If you close your housing at 90% humidity at 90 degrees, when the housing reaches 87 degrees condensation will form on its interior surfaces.