Yes, you are absolutely allowed to bring a Hydro Flask or any other brand of reusable water bottle into Disneyland and Disney California Adventure, and it is highly recommended as a way to stay hydrated while saving money. The main restriction to keep in mind is that the bottle must be empty of any prohibited liquids (like alcohol) and, most importantly, it cannot be made of glass, as glass containers are strictly banned inside the parks for safety reasons. Hydro Flasks are particularly useful because they are vacuum-insulated, meaning they can keep your water ice-cold for hours even in the intense Anaheim heat. You can refill your bottle at various water bottle filling stations located throughout the parks, or you can ask for a cup of free ice water at any quick-service dining location to pour into your flask. Just be aware that larger "gallon-sized" jugs might be cumbersome to carry and could potentially be flagged if they exceed the maximum dimensions for bags and coolers (24 inches long x 15 inches wide x 18 inches high), though standard-sized Hydro Flasks (18oz to 40oz) are never an issue.