An aircraft that buries itself into dirt or mud does not imply that it remains intact. It commences breaking up as it hits the water, but the horizontal separation of the parts has not proceeded significantly far in the approx single a/c length involved in this case and each part has sufficient kinetic energy to bury itself. I have watched an a/c bury itself into soft soil at around 600kts vertically and there was almost no above ground wreckage. The shallower the water the greater the chance of burial. It is not intact underground but a series of parts buried at a depth relating to penetrability. In my case the dummy missiles and engine were over 30 feet underground. If it enters water more than a few body lengths deep there will be lots of wreckage on the surface. It all depends on depth, angle and speed.
The KQ507 crash in Cameroun pretty much buried itself in mud and there was very little debris at ground level making discovery and recovery very difficult.
So, if the 777 went nose down into mud, we may not see much. I think I also remember in the KQ crash the recorders were 20-30 feet down too.