Ewa-Marine CD-5 Desiccant Silca Packs will help prevent condensation from forming inside a housing. Condensation will form in a housing when you take it from warm air to cooler water. Internal camera flashes being fired inside the housing will compound this problem.
Since the silica needs some time to act, give it about 10-15 minutes after closing the housing before entering the water. The CD5 is blue in color. Once saturated with moisture they turn red. CD5 can then be reactivated by drying them in the sun (only on a dry atmosphere) or in an oven. This process can be repeated up to five times.