Yes, Disney security will check your Hydro Flask (and any other reusable water bottle) during the standard bag check process at the entrance. In 2026, the primary goal of this check is to ensure that the bottle does not contain alcohol or glass, as both are strictly prohibited inside the theme parks. Security officers may ask you to open the bottle so they can smell the contents, or they might use a "vapour" detector in some locations. You are perfectly allowed to bring in a Hydro Flask filled with water, soda, or juice. It is a highly recommended practice, as Disneyland offers numerous "Bottle Filling Stations" throughout the park to keep guests hydrated for free. As long as your bottle is not made of glass—which is a safety hazard—and does not contain prohibited beverages, you will have no problem bringing it through the gates. Just be prepared to pull it out of your bag for a quick visual or "sniff" inspection by the Cast Members at the security checkpoints.