There seems to have quite some time taken to find the wreckage, have their been any statements on whether the sonar transponders worked properly, and if they do why the search was so long ?
According to my A0283 plot (sorry to label it like this, but I try to keep track of different plots and their updates including my own). On December 29th there was an official statement about searching/listening for pings around a specific coordinate and a 15 miles radius around it. If these were nautical miles then the FDR was a the edge of that radius or just outside. If they were statute miles then the recorders were outside that area.
If they were on the edge then you should have to know the effective range of the listening devices. Ranges mentioned during recent investigations are between 500-2000 meters.
During the MH370 search I read a comment which stated reduced effectiveness in shallow water. Next to the fact that in general in shallow areas there is more traffic and vessel noise. This is quite a busy area. It suprised me that only now they have closed a sector for traffic for better listening. A few days ago a big vessel was sailing right through the area (if you may believe that tracking site). So to better answer your question, we need someone who can say something about effective range in shallow water. The floor here was sand according to an official statement. Depth at the location was 30-32 meters.