Yes, you are absolutely allowed—and encouraged—to bring your own water into Disneyland and Disney California Adventure. The only strict rule is that the container must not be glass. You can bring factory-sealed plastic bottles or high-fidelity reusable bottles made of metal, plastic, or silicone. This is a significant money-saver, as bottled water inside the parks can cost upwards of $5.00 per bottle. Disneyland provides numerous "Bottle-Filling Stations" throughout the parks (identifiable in the Disneyland app) that offer chilled, filtered water. Furthermore, you can go to any Quick Service food location and ask for a "Cup of Ice Water," which they will provide for free. By bringing a reusable bottle, you can stay hydrated in the California sun without constantly standing in line or generating single-use plastic waste. Just ensure your bottle is tucked into your bag during security screening, and you'll be good to go for a magical day.