Jessica, do not put bottled water in a suitcase with anything that can be water damaged. On our cruise to Alaska during this past summer (August of 2007) my wife and I heard a story from a passenger about how bottled water had opened in a suitcase and ruined some clothes, a camera, and some other things. If you must bring bottled water on board (and this is a good idea toward keeping the overall cost of your cruise less expensive and because you are driving to your port of departure), buy a case and keep in sealed and put it in the suitcase with your beach shoes, bathing suits, t- shirts, and other materials that water cannot harm. The water on the ship is free but having a cold bottle of water when out walking around ports,and at night with your medicine is a nice treat.
Please note that after 20 cruises I have never had any problems with tap water on a cruise ship.
Hope that this helps.