I'm told “derive plonge entre deux eaux” literally means drifted plunged between 2 waters.
Selon un expert aéronautique toulousain qui a requis l'anonymat, l'élément de la voilure n'aurait pas flotté pendant plusieurs mois à la surface de l'eau mais aurait dérivé, plongé entre deux eaux à quelques mètres de profondeur.
Mrs gawbc is French so I asked her to translate this (even she thought it was a bit odd) but:
According to an aeronautical expert form Toulouse (actually a Toulousian) who wished to remain anonymous, the flaperon would not have floated for many months at the surface but would be between two areas (seas, possibly surface and bottom, or even currents?) at some metres depth
Rob