Yes, you are absolutely allowed to bring a water pack (such as a CamelBak or other hydration reservoir) into Disneyland and Disney California Adventure. In fact, it is a highly recommended strategy for staying hydrated in the California heat. When entering through security, ensure that the reservoir is either empty or contains only water; security Cast Members may ask you to open the pack for inspection. There are no size restrictions on the hydration pack itself, provided it fits within a standard backpack and does not have wheels (which are prohibited). Throughout the parks, you will find "Bottle Filling Stations" specifically designed for filtered water, often located near restrooms or in newer areas like Star Wars: Galaxy’s Edge. Additionally, any quick-service restaurant will provide you with a cup of ice water for free upon request, which you can use to refill your reservoir. Using a water pack allows you to stay mobile and avoid the high cost of bottled water (often $5 or more) while navigating long attraction queues where stopping for a drink might be inconvenient.