The FDR data will still tell them whether it flew (ie pilot commanded) into the water or crashed (ie ran out of fuel or suffered engine failure), whether it had

The FDR data will still tell them whether it flew (ie pilot commanded) into the water or crashed (ie ran out of fuel or suffered engine failure), whether it had been on autopilot, whether there was an earlier fire that knocked out communication and navigation etc. There are plenty of events where the CVR provided no useful information and the cause of the crash was determined by the FDR.