If you really like bottled water you can save some money by buying a case of 1-liter bottles at Costco or Sam's Club and distributing them in your checked luggage. Cost for 15 bottles is about $6--much cheaper than purchasing on board.
The water on the ship is perfectly safe to drink, including the tap water in your statroom, although some think the taste is not as good as what they are used to at home. The water at the drink station on Deck 9 aft is specially filtered for taste as well, so if you want the best-tasting drinking water on the ship, use the drink station. It's free.
On our last cruise, we took several bottles of water from home, then refilled the empties at the drink station as we drank them.