Silica Gel
Great desiccant for keeping moist away from your compact housing or GoPro while diving.
Comes in small sealed packages ready to use.
(small enough even for iPhone housings)
A desiccant is a hygroscopic substance that gives a state of dryness (desiccation).
Commonly encountered pre-packaged desiccants are solids that absorb water. Because of their common use in preservation, desiccant are chemically stable or chemically inert.
The most common desiccant is silica, an otherwise inert, nontoxic, water-insoluble white solid. Tens of thousands of tons are produced annually for this purpose.