Yes, Disneyland allows guests to bring Hydro Flasks and other reusable metal or plastic water bottles into the theme parks. In fact, Disney encourages guests to bring their own bottles to stay hydrated in the California sun and to reduce plastic waste. The only strict restriction regarding containers is that glass is prohibited (except for small baby food jars). Your Hydro Flask can be brought in even if it is already filled with water or other non-alcoholic beverages; however, security may ask you to open it or perform a quick check to ensure it does not contain alcohol. To make carrying a bottle easier, Disneyland has installed numerous Bottle Refill Stations throughout the parks, such as in Tomorrowland (near Autopia) and in Star Wars: Galaxy's Edge. Additionally, any "Quick Service" food location will provide you with a free cup of iced water upon request, which you can use to top off your Hydro Flask. This is a great way to save money, as a single bottle of water inside the park can cost upwards of $5.