I did email Darcy and she was kind enough to reply this evening. She said an 8-ounce water bottle should fit, but that's a pretty small water bottle. The average one sold in a store is typically 12-16 ounces. I think you'd have to get a pretty small one to fit, which kind of defeats the purpose of using it to bring water with you.
Wish this thing were just a tad bigger. Looks like a really interesting bag.