Bernd,
Buoyancy devices make little sense because air crashes are so unpredicatable.
Think of somethng like the Hudson crash... imagine a larger impact, more waves, and the tail would have broken off, then sunk. Any buoyancy devices would have dragged the CVR and FDR into the tip of the tail and they would have been dragged down with it.
Most crashes involve high-g impacts and fire, which is what CVRs and FDRs are protected against in the first place. Some do occur over water, which is why there are are ULBs.
CJ