Hi Erin!
Welcome to planDisney! I am so glad you found us to ask your question. Over the years, my husband and I have carried a wide variety of bags into the Disney theme parks.
After traveling to Disney theme parks over 80 times, I am now of the “the smaller, the better” mindset when it comes to my park bag. While I still will carry my cute Disney Dooney & Bourke backpack with my travel umbrella (for both sun AND rain), my metal water bottle, my hand sanitizer & wipes, a snack or two, & my sunglasses case, I have definitely reduced the amount that I carry!
To answer your question, any bags, coolers, or backpacks taken into the theme parks must be
no larger than 24" long x 15" wide x 18" high. You can absolutely have your backpack full of bottled water, just keep in mind that it is a bit on the heavy side.
I hope this helps you with your planning for your December Disneyland visit! Please come see us again here at planDisney with other questions that you might have!
Happy Traveling,
Tracy