Yes. The U.S. Public Health Service requires that only children who are toilet trained are permitted to enter swimming pools and spas aboard cruise ships.
Children up to 4 years old, even if not toilet trained, are welcome to enjoy the water play areas aboard each ship while wearing a swim diaper. To find out the available water play areas on your ship, view the list of pools, aqua play areas and waterslides.
Please note that swim diapers are not permitted in hot tubs, spas or any Disney Cruise Line pools or waterslides, including the AquaDuck, AquaDunk and AquaLab.