Don't worry about water on board. The ship water has been fine on my previous cruises. Just want to have a couple bottles of brand that I am use to drinking. So I can take a bottle ashore for excursions.
A case of 12 bottled water is way too much for two people but there are some folks I work with who only drink bottled water - they are missing out on flouride which is never in bottled water, and which all of us need for those pearly white teeth!
so don't worry!