Wacko Scenario
There have been such a wide range of theories, many of them reflecting a lack of aviation knowledge that I've decided there is room for one more.
Perhaps some triple 7 experts can rule out this scenario.
The big puzzle here is: what could possibly happen at cruise speed at 35000 feet that would not result in a very large and dispersed collection of pieces of airplane that would easily have been noticed in a couple of days of daylight? And if there was a high energy breakup at sea level there would be a more concentrated collection of debris.
What if:
A massive electrical disturbance that took out all of the communications and much of the control electronics right down the line. Leaving the pilots with a basic panel and difficult to handle controls.
Assume the electronics are fried - without considering how this disturbance crossed over between the several buses. So, eventually they would engage the RAT and maybe get some useful hydraulics (if needed), but the RAT electrical power would not be very useful because of damage already done.
Faced with that an experience aviator might be able to control the aircraft and ride it down with or without engine power... possibly idling. Not sure what the engine control electronics does if it loses command signals.
So, with the basic panel they keep the airplane flying - possibly gliding - watch the altimeter, and hope the standard setting for FL350 is not crazy wrong at sea level. Or maybe they even had data on more local altimeter settings. As they get to 500 feet - they taper off to V1 or thereabouts and slowly descend until they hit the sea. And then float there for a short time. As sea water flows in through a few cracks in the fuselage.
And then sadly.... the aircraft sinks.
What's missing - floatation rafts getting deployed - maybe not practical.
406 MHz beacon... why is it not being heard?
Could this scenario require manual deployment?
Many years ago when I was active in small aircraft flying and airline and military avionics design the activation rate of ELTs was very poor in many crashes - the TCO got revised and the g switch specs and orientation made a great improvement. But automatic activation on nasty landings would make a lot of people unhappy.
So - could a 777 water landing leave the g detect switches untriggered?
Crazy? Impossible? Or just implausible.
So far the limited fact set seems to not produce many possible solutions.
In this scenario there might be an almost total absence of debris.
Enough...