Originally Posted by You
Most drydocks are done to maintain mechanical systems that can't be done while at sea. They will usually do some maintenance on rooms, public areas, etc. Wait until an announcement from Celebrity, everything else will be just rumor and Celebrity's rumor mill is very poor.
When a ship goes into drydock, it is exclusively to maintain (1) the hull, (2) segments of piping carrying sea water between their hull penetration and the first valve inside the hull, including the valve, and (3) external components such as propulsion pods and external shaft bearings that have watertight access points located underwater. For other maintenance that requires a shipyard's support, the ship will simply pull up to a normal pier (sometimes called a "wet dock") at the shipyard where the cranes and other yard equipment have full access above the waterline.
A cruise line will not put a ship into drydock unnecessarily because it is a very expensive and risky process. Before the ship goes into drydock, the shipyard must configure keel blocks and stabilizers very precisely to support the ship and keep her upright without putting excessive stresses on the keel or hull. When the ship moves into drydock, the drydocking team uses very precise optical instruments in conjunction with carefully tensioned guy lines to maintain the position and alignment of the ship over those blocks and stabilizers as they pump the water out of the drydock. If anything goes wrong with this part of the process, the supports can damage or puncture the hull in a way that would render it unsalvageable. The ship also must shut down all systems that depend upon sea water for cooling or water supply before the water drops below the respective penetration, so the shipyard must provide temporary external sources of electrical power, potable water, sewage disposal, and water to the ship's fire mains while the ship is actually in drydock. When the ship leaves drydock, all of this process must be reversed. Additionally, it usually is not possible to perform other work on the day when the ship moves into drydock and when the ship moves out of drydock because those evolutions render the ship unaccessible for that long. Thus, an unnecessary drydocking would protract a yard visit, and the associated loss of passenger revenue, by two days.
That said, you are absolutely correct in saying that ships usually receive other maintenance and repairs -- and often modifications as well -- while in drydock. Time spent in a shipyard is time when a ship is not generating revenue, so the cruise line will always seek to minimize that time by maximizing the work actually accomplished. Maintenance and repairs that the ship's crew cannot do while the ship is at operating, including refurbishing of public rooms, are always the first priority whenever a ship is in a shipyard, of course, but the cruise line's goal is always to accomplish as much as possible while the ship is out of service. The difference is only that the cruise line will not protract a yard visit to do work that the crew can do while the ship continues normal operations.