I think it depends on how picky you are about your water. The First time we cruised we saw people carrying flats of water bottles on board and I thought, "Oh no...the water must taste bad on the ship." It wasn't too bad but I have a tendency to visualize where the water is coming from when I drink it and found myself gagging every time I drank from the tap.
So, last month for our cruise, we brought two packages of water bottles. My husband carried one and I carried the other. The bad part is going through check-in and having to lug the water. But after you get on board, go straight to your room and drop off your carry-ons along with your water.
I drink ALOT of water, so we had already depleted our supply when we arrived in St. Thomas. At the dock, there was a small grocery/misc. store. We stocked up on water again and it lasted until the end of the cruise.
A little trouble, but SO worth it in my opinion.