@UA
I don't know either, whether you can comment on some of the issues?
Do we even know where the debris field is?
What's the resolution of the sonar? How easy or difficult is it to distinguish a piece of 'man-made' debris from the rock next to it?
It's unlikely the FDR and CVR were thrown clear from the rest of the wreckage... more likely they're still stuck in some of the aircraft structure. So much the same question... you're not looking for rectangular boxes, but for the most likely parts of the wreckage. Can the kind of sonar you mention find those?
You obviously would not send a ROV down until you had at least some kind of target, no?
CJ