The salts in seawater form an elecrolyte that corrodes any exposed metal alloy (in practice all used metals, it is very rare to find any that is 100% pure), or even pure metals if there are different types in close proximity like PCB-s. This is a rather slow process, but if oxygen (air) is introduced, it speeds up dramatically, as fresh surfaces created by the electrolytic process are immediately oxidised, and the oxide film amplifies the electrolitc process. Any electronic equipment accidentally or otherwise immersed in seawater needs to be either rinsed thoroughly with distilled water immediately, or left in freshwater that protects it from air. Even leaving it immersed in salt water is better than exposing it to air. While the FDR memory chips (actually the whole memory module) is sealed in plastic and resin, the contacts and supporting circuitry will be corroded if this precaution is not taken, and readout will become much more complicated, requiring the disassembly of the memory module. Much easier and safer to keep it immersed, and have it cleaned and dried in the lab.