Why is the FDR and CVR put into water again after a water crash?
This is actually a fairly standard thing to do with electronics that have been submerged: They are placed in water (ideally fresh, clean water) to both delay the onset of corrosion and dilute any salts or other chemicals that they came in contact with while submerged.

When you remove electronics from water and let them dry out they begin to form corrosion on all the little exposed bits of metal - chip legs, solder joints, capacitors, etc. - which can badly damage the device when it is powered back on.
Keeping the device submerged in water delays the onset of this corrosion because the dissolved oxygen in the water is lower than the oxygen in the air that would be hitting those parts as they dry out.

The Cockpit Voice Recorder and Flight Data Recorder are water resistant, but they are not necessarily waterproof (particularly after a crash, which may puncture their casings or damage seals), so they get placed in a container of fresh water for their trip to the NTSB's lab.
When the still-immersed equipment arrives at the NTSB laboratory it can be examined and dried in a controlled environment, maximizing the chance to recover data.


  For more gory details check out The NTSB Flight Data Recorder Handbook for Aviation Accident Investigations.




Bonus Factoid: Archaeological recoveries from bodies of water use similar procedures, for similar reasons: If you bring up a clay pot that's been sitting at the bottom of a lake for a thousand years and let it air-dry naturally there's a good chance it will crack and be destroyed.
In archaeological recoveries the usual practice is to keep the item "waterlogged, ideally in waters associated with the original context" (i.e. "in a bucket of water from where you got the thing").