You are allowed to bring a "reasonable" amount. The definition of "reasonable" can vary.
Also, in some ports, the porters won't accept a naked case of water like they used to. It seems too many were breaking when they went down luggage chutes at some of the ports.
If you threw the case of water in a duffel bag, they had no problem. I guess out of sight, out of mind. Plus it was an extra buck or two in tips.
You can also buy water in the ports and bring it back on.