RickI wrote: a hot camera in a very small housing will fog under some conditions.
It is not the temperature of the camera that causes the fogging.
It is the dew point of the air you trapped in the housing when you closed it and exposing the housing to temps below the dew point.
Here is the math.
Beautiful sunny day on Miami Beach today = 80F @ 70% relative humidity = 70F Dew Point
Water Temp 78F = NO Fogging
If the water went to 70 F, then the air within the housing would be chilled to its DEW POINT and the water would condense (or form DEW) on the housing.
So, all one has to do is reduce the dew point of the air within the housing to a pint where it is below the water temperature and condensation/fogging cannot occur.
Dessicants can reduce this but take time to adsorb the moisture from the air trapped within the housing. Think an hour or hours, not minutes.
The interior of a refrigerator, freezer has low relative humidity. No need to cold soak your camera either. If you "scoop" some air from the freezer and close the housing to trap the air, you will have very dry air with a very low dew point trapped in the housing and no fog problems down to water temps near freezing.
Same if you "fill" the Go Pro housing in front of an air conditioner vent.
Physics!