Thank you guys to remind me Archimedes principle. But I'm still not conviced. In AF447 case the fin was floating because it's an hollow object (mostly composite) "almost intact" and nothing was attached to it while in QZ8501 case there is quite an amount of structure attached. Also the rudder is partially ripped off, this don't help the floatation (water filling).
But maybe the proposed mapping is not quite accurate. Let's wait the official report.