I can see this thread filling up with what-if ideas and blasts from the past faster than you can say FDR, so here's what the first hit of a Google search on "airbus floating FDR" brings up (see pdf file pp. 19-29):
Flight Data Recovery - Time for evolutions
Haven't found a similar article from other aircraft manufacturers but I'm sure there will be a view from Boeing on this topic at some point in the not too distant future.