I have used this technique in situation wherein I cannot find a 'dry room' in a boat. very effective.
First U need a transparent plastic bag, about the size of a large pillow. then put the cam & housing inside the bag, squeeze out the air inside, fill the bag with scuba air halfway, then mount cam & close the housing.
easier said than done. practice, practice practice.
the fluid that came with the housing... great mask defogger