Dwalton,
The silicon bags are there to absort moisture in the air trapped inside of the housing. This helps reduce the chance of condensation in the housing by absorbing moisture as chances are, you'll have seeled the housing in a warm, humid climate (ie on holiday). When you take the camers in to the water, which is normally cooler depending on where you are in the world, then there is a risk of the cooler temperatures causing condensation inside the housing (as the walls of the housing cool, they act as a condensor). That's where the silicon bags come in, they help to absorb the moisure in the trapped air to minimise this problem. I don't bother with them personally as the size of the
housing and camera system I use would mean I'd have to half fill the housing for them to work with any effect. In a small hosuing, they are worth having as a precaution, especially if you are using it in water with a higher temperature difference (ie not a problem in the warm caribbean waters, but it will be a problem in the cooler atlantic waters). The best thing to do though is seal the housing in an air conditioned room if possible so the humidity inside the housing is low when yuou seal it.
Darren