There are juices, tea, lemonade or fruit punch, milk, and coffee available free of charge at the buffet area throughout the day. There really is no need to purchase the soda package, unless you just want to. Another suggestion is to bring something like the Crystal Light packages or something similar you can mix with water. The water on board is as clean, if not cleaner than bottled water. You could bring some water bottles or thermo cups and fill those up with ship water and ice to go on shore or to have around by the pool.
Hope this helps a bit