Isn't that the price they charge anywhere on the ship for the water. I know a Soda is $1.75 in the stateroom or if someone brings it to you somewhere else on the ship.
It is interesting point on the cabin Stewart. I wonder (and hope) if he does get a cut for stocking the frig. It is added work on his part.
Our cabin Stewart had a soda in the room for my wife every evening. He noticed her bring one in on the 2nd day and knew we had soda cards. We were just below the aft deck bar so it was not much of a trip, but it was one that got he $$ when we tipped him several times during the cruise.