Yes, you absolutely can and should bring your own water into Disney World and Disneyland. Disney has a very generous policy regarding outside food and non-alcoholic beverages. You are permitted to bring in your own reusable water bottles (plastic, metal, or silicone), provided they are not made of glass, as glass is strictly prohibited for safety reasons. Bringing your own water is a major money-saver, as a single bottle of water inside the parks can cost upwards of $5.50. Additionally, every "Quick Service" dining location in the parks is required to provide a cup of ice water for free upon request, which you can use to refill your bottle throughout the day. You can also bring a small cooler to keep your drinks cold, as long as it does not exceed 24" x 15" x 18" and does not contain loose ice (reusable ice packs are allowed). Staying hydrated is the best way to prevent heat exhaustion in the intense Florida or California sun.