According to frequent discussions on Reddit and official 2026 Disneyland Resort guidelines, you are absolutely allowed to bring a Hydro Flask or any other reusable water bottle into the parks. The only strict restriction regarding containers is that they must not be glass, as glass is a safety hazard in the theme park environment. Many "Redditors" suggest bringing a 21oz or 32oz bottle rather than the massive 64oz "growlers" to save on weight while walking 10+ miles a day. Disneyland has significantly expanded its number of bottle-filling stations in areas like Galaxy’s Edge, Tomorrowland, and Avengers Campus to encourage sustainability. Additionally, you can walk into any Quick Service restaurant and ask for a free cup of ice water to refill your Hydro Flask. Just be aware that during security screening, you may be asked to open the bottle if it contains liquid, though most modern "contactless" scanners can identify the contents without a manual check.