Apple does not manufacture waterproof cases for its products. Waterproof cases for Apple's iPhones are available from other manufacturers, such as LifeProof, Hitcase and Lifebox.
Apple does sell waterproof cases through its stores, but they are not manufactured by Apple. It is important to research waterproof cases before purchasing one, as there are some that do not perform as well as others. Check all product features and read all instructions and warnings, such as maximum depth and duration that can be withstood, before making a decision. Products labeled as water-resistant are not the same as products that are waterproof, and they do not perform as well.