Never thought of filling an entire water bottle either, as my preference is directly from the mouth, but if filling a glass or water bottle, a mixture of all the Mistresses spit mixed together would I am sure make for a wonderful tasting beverage.
I like how You mentioned that You just aim straight for the face...or wherever. As much as I love to be covered in a Mistresses spit, I would bathe in it if I could, I especially like it in my mouth and orifices, as I like to be filled with spit as much as I enjoy being covered in it.