Regular casts should stay dry at all times to prevent skin irritation or infection. To prevent water damage during showers and baths, wrap the cast in cling film and seal it with duct tape. Fiberglass casts can also have a waterproof liner but that costs extra and you should ask the doctor if it's okay for the cast to get wet. In case a cast does get wet, dry the inside with the hairdryer set on low heat.