The water bottle in the little watercolor kits is usually too small to be of much use. I carry one or two very flat plastic bottles that hold a few ounces of water. They are small enough to carry in my pocket, and originally held some kind of lotion. The only requirement here is that the tops screw down tightly and do no leak. In fact, all of the materials described so far, with the exception of the watercolor paper, can be carried in a couple of roomy pockets---a fact greatly to be appreciated when you ascend the Acropolis to paint the Parthenon (if you are lucky enough to do so)!