No matter what the water situation is on board, I usually only drink it in iced tea. That's just my preference.
Water is a big item near most ports, so I wouldn't bother packing any unless you'll initially be at sea many days. If you're able, pick up some prior to embarking. It is still not cheap in port, but less costly than on board.
I figure, I'm on a cruise to have a good time. I drink a lot of water. I'm not going to whine about spending a few extra bucks on bottled drinking water to keep myself happy.