We were amazed while boarding the Glory this past August that people actually had two and three cases of water strapped to their wheeled carry-ons. This water was in plastic wrapped cases just like you get it from the store. They were not stopped.
MARINER OF THE SEAS 10/31/2004
Fantasy 1991 AND 2002
Yes you can bring water onboard but supposedly in a reasonable amount. I too am a huge water drinker but what we do is just bring on a few bottles and re-fill them from the tap on the ship. The water there is a clean and pure as the bottled stuff and you cannot taste a differance.
If you do need to buy some bottled water, do it in port, where you can usually buy them for $1 versus $2 each on the ship. I also brought along a little insulated cooler bottle (like a sports bottle) and filled it with ice from the room (ask your steward to bring you ice each night) then fill it with water from the taps on the Lido deck. That way, my water stayed very cold for at least several hours.
Just back from the Carnaval Paradise,. there was a UNITED WAY fundraiser going on while we boarded in Miami. They were selling bottled water and pop by the case and it was delivered directly to your state room at a very reasonable price (half of ship price). It saved us the hastle of packing and carrying it all. I don't know if this is usual or not. But if you are a big water drinker bottled water is a MUST. The on ship water is safe to drink but it definetly has a clorine taste.