Yes, you are absolutely permitted to bring a full reusable water bottle into Disneyland Park and Disney California Adventure. Unlike airports, Disney security allows liquids as long as they are not alcohol and are not in glass containers. Bringing a pre-filled bottle is a smart way to stay hydrated immediately upon entry. To save money throughout the day, you can refill your bottle at various "Bottle Filling Stations" located near many restrooms or at any Quick Service restaurant, where you can ask for a free cup of ice water to pour into your own container. Just ensure your bottle is made of plastic, stainless steel, or aluminum; glass is strictly prohibited for safety reasons (except for small baby food jars). Also, keep in mind that if you are using a cooler to carry your drinks, it must be soft-sided and no larger than 24" long x 15" wide x 18" high.