well, this might be a little unconventional but:
has anyone tried to use some parapsychologist technology to show possible location of the plane? use several well qualified and independently working people to get several locations spotted, select where the density of spots is higghest and check there. This is a known and used way of founding things on the ground and
AFAIK it works as well over water but it may have to be assisted by better maps of the bottom surface?
there are people capable of doing such things - with good results, even it there is much more people who do not believe this. I do not want to start a - i believe - i do not believe - discussion here. Just pointing out a possible way that can be exercised even before start of the coming search phase 4.