I have a small, barebones one, as I don't like things on my back when I'm mountainbiking (the only time I use it). That being said, +1 to a small zippered pocket somewhere on it.
They come from small to huge, so it depends on what else you might want to carry in it. And keep in mind water is heavy. Stupid sounding, I know, but the bigger the bladder, the heavier the pack when it's full.