Yes, you are permitted to bring water flasks and reusable water bottles into Disneyland, provided they are not made of glass. Disney's official policy allows guests to bring outside food and non-alcoholic beverages into the parks for self-consumption, as long as they do not require heating, processing, or refrigeration. Reusable plastic or metal flasks (like Hydro Flasks or Yetis) are highly recommended because Disneyland and Disney California Adventure have numerous "Bottle Filling Stations" located near restrooms and in newer lands like Star Wars: Galaxy's Edge. These stations provide filtered, chilled water for free, which is a significant cost-saver compared to buying bottled water inside the park. Just ensure your flask is not exceptionally large; bags and coolers must be smaller than 24" long x 15" wide x 18" high. Additionally, "loose ice" is technically prohibited at security checkpoints, so it is best to fill your flask with ice at your hotel or use reusable ice packs if you are bringing a small cooler bag.