Electronics
Trouble is, in a vacuum environment it is easy to hang electronic stuff on the outside - Radios/Mobiles/PDA's/Cameras/data loggers.
You need to know in advance if you intend swimming with them, especially in chemically reactive and therefore conductive saline solutions...
As for the "Why", I'm not sure, but a 10-tonne mass of chemically-buffered water is a good way to soak up chemical activity, IF it exists.
After the first couple of serious missions, then we'll learn if we need this sort of precaution...