repeating an oft mentioned fact
most bottled water is locally sourced water that is purified by the reverse osmosis method. Often some salts are added back in for flavor.
most cruise ships produce their fresh water from seawater via reverse osmosis systems, sometimes augmented by actual heat distillation systems. Additionally the 'rules' require testing ship's water supplies MUCH more rigorously than 'bottled water' producers ....
in other words the tap water on the ship is typically as and often more pure than typical bottled water.
So unless paying for 'spring water' <and even then>, bring some sport bottles and refill as needed for free ...reduce your plastic footprint ... it is the same water they are using for your ice, soft drinks, coffee, tea and cooking . . .
On my last RCL cruise (Nov) there were big dispensers of iced water at every bar for you to fill up for free .... and btw if you have the delux drink package bottled spring water is included