Yes, you are permitted to bring insulated water bottles, such as Hydro Flasks or Yetis, into Disneyland and California Adventure. The only strict rule is that they must not be made of glass, as glass containers are prohibited for safety reasons (with the exception of small baby food jars). Security will inspect the bottle at the checkpoint to ensure it does not contain prohibited substances; it is often easier to bring the bottle empty and fill it once inside at one of the many "Hydration Stations" or water fountains. Bringing an insulated bottle is a popular tip among regulars, as it keeps water cold during the hot Anaheim afternoons and helps avoid the $5+ cost of bottled water in the parks. Note that while the bottle itself is allowed, loose ice cubes in coolers are restricted, so using the bottle's insulation to keep pre-chilled water cold is the best strategy.