deSitter
My understanding is that hydraulics are not immediate. Whether nose first or belly on, there is a hole created whether the surface is Dirt or water. Water doesn't get hydraulic until some of the energy of impact dissipates, and the displaced water has some place to go, since water is basically incompressible when contained. The 447 is a very large essentially hollow cylinder with concentrations of mass where necessary. There are two volumes inside the tube, cargo and cabin. She hit flat, so the cargo hold pressurized quickly to a high value (for a not so very long time, however).
Early on, I noticed the crew rest capsule appeared to be a lamination of two sheets of aluminum with an adhesive layer of Phenolic resin. The separation of the aluminum sheets appeared to have signature of gas compression/release, acting as a miniature pressure chamber, a la the hold and cabin. Where BJ-ENG noticed a floor former on the seabed, I noticed what looked like a casting for some machinery structure, in other words, cargo.
I think that an a/c is made of millions of parts, and when undergoing such an impact as 447 has, these fastenings one to the other are virtually (essentially??) irrelevant, save some very strong assemblies, Gear, Engines, Wing box, etc.
So what happens is the a/c comes completely apart due to chaotic and multiple stressors, for several quantifiable reasons, reasons beyond my ability to understand, to be sure. The theory that the cabin could briefly have acted as a gigantic "airbag" is seductive, though the fuselage is probably the weakest structure on the airplane. I can see how some contents could have escaped obliteration, though placed in proximity with other parts that were destroyed.
I think the cause of the accident is somewhere in the simple understanding of systems weaknesses, and probably some quirks of fate that ganged up on our 447.