The Australian Government has described the sat image as credible and that the object yet unidentified appears to bear signs of it being awash or semi submerged. Some weight must be attached to such a statement given the gravity of the situation as a whole. There is much speculation as to an object of this size, reported as 24m, remaining on or close to the surface after such a long lapse of time, in the event it may previously have formed part of the aircraft structure.
The following link details the materials and technology applied in the design and construction of the B777 and may give a usefull insight as to the likelihood of wreckage that may be discovered.
http://www.southampton.ac.uk/~jps7/A...0materials.pdf