Most ships will have the facility to order bottles of water through your cabin steward. if you leave the order in the morning it will normally be there by the afternoo.
I have been on 5 cruises.All the ships have had bottles of water in the cabin which if you use are charged to your bill.
All water in your cabin and from the catering areas on the ships of the major cruise lines is drinkable. If you feel that you must have bottled water or you need it for some other reason then buy it by all means, the cruise companies will love you all the more for the purchase, but the water served at the tableand in your cabin and all the ice all comes from the ships system and is fine.....Nei.