Whether a toddler can swim on a cruise ship depends entirely on their toilet training status and the specific facilities of the cruise line. Due to strict United States Public Health (USPH) regulations, children who are not 100% toilet trained—including those wearing "swim diapers"—are legally prohibited from entering the main swimming pools or hot tubs on most cruise ships to prevent the spread of waterborne illnesses. However, many family-oriented cruise lines, such as Disney Cruise Line, Royal Caribbean, and Carnival, have invested in specially designed "Splash Pads" or "Baby Splash Zones." These areas use a "single-pass" water system or advanced filtration that meets health standards for children in diapers, allowing toddlers to play in the water safely. If you are traveling on a more adult-oriented line like Celebrity or Viking, you may find no water facilities for non-potty-trained toddlers at all. It is essential to check your specific ship's deck plan before booking, as being turned away from the pool on a hot sea day can be a major disappointment for a toddler. Once a child is fully out of diapers, they are free to use almost all onboard aquatic facilities alongside their parents.