Fuel dump
Apologies for continuing to talk about this. I don't think the "white cloud" was a "rooster tail" because it happened a while after the sea wall had been crossed. At a rough guess, the aircraft might have had of the order of ten or twenty tons of fuel still on board (this is a totally uneducated guess, sorry). From a quick search, I see a B777 will burn 40 tons of fuel per hour at full power, so the low pressure pumps can supply at least 2/3 of a ton per minute. It seems that this would not be enough to generate a really large cloud of fuel in the tens of seconds after the sea-wall impact, but before the astonishing semi-airborne 360 spin (the cloud occurred before this).
But the wing tanks could have been severely ruptured at the time the engines were broken off, or due to the hard landing after the tail was lost. In that case, tons of fuel would be spilt rapidly while the plane was still traveling at speed.
The above is speculation, but the remarkable fact that there was not a catastrophic fire seems worth thinking about.