The density of sweater changes little with depth (about 1024 at surface to 1050 kg/m3 at 5000m). With pressure increasing at 1 atmosphere every 10m of depth, all air -pockets will be squeezed out or imploded as wreckage sinks. The aeroplane construction materials are pretty well all denser than seawater at any depth, so when filled with water, the entire wreckage will sink (excepting perhaps a few lighter items such as spongy seatbacks, personal effects, garbage etc which might escape prior to sinking).
They are looking at the right depth, but it appears in the wrong place....
I too would like to see the detailed calculations done by Inmarsat. If they had been released much earlier, just maybe the search would not have wasted time in the far southern Indian Ocean; just maybe there would have been a much greater opportunity to locate the FDR pinger.........