Why are FDRs and CVRs recovered from underwater transported to where they are analysed in water?
Aviation StackExchange: Why is the FDR and CVR put into water again after a water crash?
The article in the above link links to
NTSB: Flight Data Recorder Handbook for Aviation Accident Investigations (PDF)
The NTSB Handbook in section 3.9 says:
If the FDR is recovered in water, it shall immediately be packed in water (fresh, if possible) and not be allowed to dry out. Packaging may be accomplished by sealing the recorder (in water) inside a plastic beverage container with silicon adhesive or a similar sealant. Contact the Chief of the Vehicle Recorder Division for further assistance.
This demonstrates that for the NTSB at least (and likely for other organisations), this is standard practice, even though the handbook does not detail why the procedure should be followed. There is plenty of uninformed comment on the Internet giving unreferenced opinions on why this procedure is followed. Most Internet search engines will allow you to find such opinions.