NCL sells premium and standard bottle water in the dining rooms. If you don't want bottled water when dining, standard ship tap water is provided. It's clean and very good, just as good as a $3.50 bottled water.
On excurisons, NCL will offer bottled water at about $3.50 per bottle when exiting the ship to port.
Remember once off the ship you can buy all the bottled sealed water you want for about a $1.00 per bottle in any port of call. Plus you can bring it back on board. Consider bringing back a 6 pack of water on board, only if the tap water really bothers you.
Enjoy the cruise.