What can I do to prevent fogging within the GoPro case when I take it out on the water?
An important assumption that you must confirm is that the condensation is due to moisture present inside the camera at sealing time and is NOT caused by water ingress during operation. If the camera is sucking in water, then you have a problem that must be repaired.
If the condensation is due to moisture in the air in the camera it usually occurs because the outer case is cooled to below the dew point of the internal air. You overcome this by lowering the dew point of the internal air, which you do by drying it.
You can dry air with a dessicant OR by cooling it to below its dew point so that water precipitates out. So ...
Some mix of:
1 - Use a desiccant inside the camera. silica gel is one. There are others available commercially. Wikipedia is useful They note -
- . Some commonly used desiccants are: silica gel, activated charcoal, calcium sulfate, calcium chloride, montmorillonite clay, and molecular sieves. I have been very impressed with both Montmorillonite_clay and Calcium Chloride.. Both of these can make a mess by liberating liquid water after capture if arranged poorly BUT a properly designed system can capture and retain far more water than you should have inside your housing.
2 - Close case when air is as cold as possible (low RH for a given situation).
If you seal the case at home or in an environment when power is available you could use a modest peltier cooler to achieve lower temperatures than you will otherwise be likely to get.
3 - Use dry air when sealing. eg pump air through a dessicant with a squeeze bulb or similar and flush inside of camera with this air before sealing. Doing this when the air is cool is a bonus.
Choice 1 can be used in conjunction with others.
Choice 3 is arguably the most "real" one as it provides air that is as dry as you can reasonably make it. Even better if it is cool when sealed as the RH will drop as the temperature rises.