The only thing I can suggest is you put a few water bowls in the cage, so in case she tips one over, or fowls one, there's always the other one available.
Other than that, (and it's probably obvious) just put the fresh water in the cage minutes before you leave and give fresh water the minute you get back.
And have fun (don't worry) while you are on your trip! One week for a snake really isn't that horrible to go without food/water. I know that I've never been able to watch my snakes all the time, but at times during shed cycles they usually just curl up in a hidebox for a week and I never see them out, even to get water, so if they can do it during a shed cycle, it shouldn't be a problem other times too.