While we endorse cruise line rules in the forum, I can tell you that from what I've observed they don't get too excited if a passenger brings bottled water or a couple of six-packs of soda aboard. Just have it in your carry-on and it will likely be ignored. Booze is what they are focused on. I've seen many passengers embark with water and soda in the past several RCI cruises. Just be prepared to lose it if Barney Fife happens to be on duty at the x-ray machine.