Fdr Data
My Avionics crew and myself had a interesting discussion on this topic and the best solution we could come up with was a exterior mounted FDR (a slave unite to the FDR the size could be very small, Keep in mind a thumb drive is smaller than the ELT or ULB). It would have a pressure switch to release from the airframe at say 100-200 feet water depth and float to the surface with ULB and ELT transmissions.
Cost is the major factor but would be a solution to the issue we ponder.