I am still amazed that no-one in authority appears to have mentioned checking seismic recording equipment. A couple of previous posters here have mentioned it.
Seismic equipment can throw up a record of the tiniest earth tremor - and 200+ tonnes of aircraft hitting land or water at 500, 600, or 800kmh must have produced a measurable shock wave (if it has crashed, as we must presume, is the more likely scenario).
Maybe the only way there would be little shock wave is if the aircraft stalled and flew into the water nearly level, a-la AF447, at just over 200kmh.
In which case, surely there would be a pile of floating debris? 10% of the B777 construction is composite, I've been informed, so would that style of composite float? Seating, bodies, carry-on luggage, a lot must float for days?