ikohawaii, yes you do need to bring some water...but for the plane more than the ship. I make it a point to bring at least one bottle of water per person on the plane. The flight attendants aren't always serving liquids when I get thirsty and I've found that one cause of jet lag is dehydration.
FYI, on our trip to MCO in May, we got stuck on the plane, ON THE GROUND in Dallas for 3 hours without any service since we might be leaving any minute. The water was a lifesaver.
Besides, if you bring several water bottles on the plane, you'll have them to fill up on the ship.
One thing we bought on the ship that we love was a neck strap that tightens around the neck of a water bottle. Made it REALLY easy to carry the bottle if we went shopping and on excursions. They were selling the water with the straps (and the straps separately) near where we were gathering for the excursion.
DD says the water on Deck 9 drink station and all the water served in the restaurants is filtered and tastes great.