Machinbird;
"Like a stone...", yes, IIRC, the entire SSFDR weighs around 15lbs, and the CSMU is heavily-clad with steel, thermal layers, etc, etc and isn't going to float. It is certified for 30days @ 6000m so hopefully longer at 3900m. Even if compromised, the chips may be in data-retrievable condition.
Chris;
Congratulations on London's "distraction" - a bit of good news among the huge dregs of the other kind.
Re the QAR, it is mounted in the main Equipment rack below the cockpit floor. There are many variations on such an installation. Someone can correct the information if they know for sure, but it is possibly either a 256Mb or more likely a 512Mb PCMCIA card. Given the current impression we have of the nature of the impact and the location of the equipment rack, the survival of the QAR intact is questionable but always worth finding/retrieving as are any cards/memory units from the MCDUs, etc.
Here is a simple diagram of a representative aft fuselage and tail cone structure:
