Yes, you are absolutely allowed to bring water bottles into both Disneyland Park and Disney California Adventure. In fact, Disney encourages staying hydrated and provides numerous "Bottle-Filling Stations" throughout the resorts, which can be located easily using the official Disneyland app. The primary restriction is that the container must not be made of glass, as glass is prohibited for safety reasons. You can bring either disposable plastic bottles or reusable metal/plastic flasks. Additionally, if you are bringing a cooler to keep your drinks cold, it must be within the size limit (24" x 15" x 18") and cannot contain loose ice—reusable ice packs are required instead. For those who don't want to carry a bottle, any quick-service dining location in the park will provide a cup of ice water for free upon request, ensuring that guests can remain comfortable during the warm Southern California afternoons.