I thnk that is why they are leaving all the sea water inside the Engine Room at present, to stop corossion.
When they get to the repair yard they will start by pumping out all the Bilge Oil and sump oil floating on top,( Lets hope the engineers had all the sounding cocks properly closed) washing everything down with fresh water and presumably pump out the Sea water at the same time bringing down the water level one deck at a time. they will then pressure wash everything with distilled water, rinse it off with Electrical Solvent cleaner and put big fan heaters on to dry everything out before it corrodes, I dont know the layout of these ships, but I imagine most of the important stuff is protected by Sea Water at present. Tropical Temperature Sea Water has a very low Oxygen level, ( Oxygen solubility in water varies inversely to temperature, that is why cold water fish can grow larger) so corosion will be extremely low below the water level which was reported as 18 metres.
The worst corossion will be "between wind and water" around 18 metres above the bottom of the Engine Room. i dont suppose there is too much equipment up there.
The important thing is not to have anything exposed to the air and wet with salt water at the same time, as the Sea water level goes down, the yard workers will be drenching everthing in Fresh water as soon as it becomes visible.
They might even keep the water level inside the engine room the same until they have flushed all the salt out, but that would use alot of Fresh water!!
Probably one of the main considerations in not doing the work in Egypt was the high cost of Fresh Water, they will probably need thousands of tons of it and probably hundreds of tons of distilled water.
As a side issue water may not be as damaging as you think, in one Environmentally Friendly Norwegian Dry Dck I was in they cleaned our Alternators by washing them with distilled water and drying them out with fan heaters rather than using Electrical Solvent Cleaners such as Armaclean or Electrosolve.
Most of the stuff should be salvageable, things like switches meters and small breakers, and electronics will not be worth the Labour cost and delay to the ship of stripping down, cleaning, rebuilding and testing, it will be quicker and cheaper to replace them, but things like large Electric Motors, Alternators the Bus Bars of the switchboard etc will just need to be cleaned up, unless they were "live" when they drowned, same with the engines, and most of the other machinery. I imagine Maersk will try to get a lot of stuff on "Service Exchange" to speed things up.
The repair cost and time out of service will still be a huge financial hit for the owners, P&I Club, and Hull & Machinery Insurers and they will all be after the Engineer's Blood for leaving the Watertight Doors open on Stand By.
I have talked the Engineers on my ship and we can not understand this, they must have had a series of Alarms, first for thruster failure, then all the tunnel well bilges would have filled and gone into alarm sequentially, then the ER bilges would have filled and alarmed sequentially and the Machinery spaces filled to a depth of 18 metres and nobody, either on the Bridge or MCR closed the WT door either remotely or locally. It is a basic requirement of SOLAS that a watertight door can be remotely operated from a safe position above any potential flooding and that the door will close against a flood of water coming through it. However large the hole in the thruster tunnel, the flooding rate would have been restricted by the water tight door opening, which is small in comparison to the size of the Engine Room which is literally Cathedral size on that sort of ship.
Maybe the Engineers were just overwhelmed by the number of alarms and assumed the Alarm system had failed?
It is always better to ask a stupid question than to do a stupid thing.