Hiya Owen!
Water you doing soon?! You’re going to The Most Magical Place on Earth, that’s what! I am elated to hEAR you’re getting ready for your trip and want to thank you for bringing your question to us.
Hydration is extremely important and you can absolutely bring a water-filled hydration pack into the Theme Parks. The only parameter to look out for is that Guests are welcome to bring
backpacks of any sort that are no bigger than 24 inches long by 15 inches wide by 18 inches high. Typically hydration packs aren't that big, but I thought I’d mention it just in case.
Owen, you will also find hydration stations at each Theme Park. They have a drain under them so there's no mess as you refill. There is one at
Magic Kingdom Park at Cosmic Ray's. At
Disney's Hollywood Studios you will find it where the restrooms are in Galaxy's Edge, near the exit of Smugglers Run, and by the line at Slinky Dog Dash. At
EPCOT, you will find one at the Starbucks in the World Showcase. At
Disney's Animal Kingdom Theme Park, there is one by the lines of Na’vi River and Avatar Flight of Passage and Expedition Everest. The hydration stations are all contactless and cleaned regularly.
You can also ask for free cups of water (with or without ice) at many places around the Theme Parks. Many of the
Quick Service locations offer them; all you have to do is ask. If the location doesn’t offer cups of water, a Cast Member will be happy to point you in the right direction.
I hope you have an enchanting time, Owen!
Aena