On most of our previous cruises we almost always drank tap water. Very safe and it never tasted all that bad. My wife would fill up bottles for shore excursions etc. I don't care much for the taste of chlorine though, especially in my coffee. Bottled water was preferable for me except it was rather expensive and my wife would give me a bad time when I bought them on board. We cruise on three lines, Princess, HAL and Regent. Regent gives us bottled water for free, so I really get spoiled on them.
Recently though, I discovered that on Princess we could order a case of 12, waiting in the cabin, for .57 cents per bottle---500 ml. It is my understanding we can get the same on board by the case anytime for the same price, but a case for two weeks is more than enough. For a month cruise I would order two cases. I'm not about to pack an entire case on board in my luggage, besides some lines are beginning to prohibit that practice.