Becky,
Thanks for your question!
Of course you can bring a water bottle in with you to the parks. Staying hydrated, especially in the summer, is essential. Disney definitely welcomes you to bring in something to stay healthy and happy during your day at the parks.
There are some restrictions to what you bring in when it comes to
coolers, like they cannot have loose ice or dry ice. They also have to be no larger than 24" long x 15" wide x 18" high. I know you just said water bottles, but I thought that in case you were also considering a cooler, you might want to know.
By the way, a lot of people don't know this, but any of the quick service restaurants will gladly give you a glass of water, no charge. This is an easy way to keep yourself hydrated, though it can be a bit tedious for refilling your bottle. Water fountains are typically located adjacent to most every restroom, and that will make it easier to replenish your supply.
I hope that this helps. Please do send us any follow-up questions.
Have a Magical Vacation!
Doug