Considering the reported depths, I think that it is doubtful that anything that made it to the bottom would have any flotation left. The squeeze pressures would flatten every air pore.
Think about the fire extinguishers, oxygen bottles and similar items. There is a possibility they could withstand that pressure, and on their own have positive buoyancy. In fact, something that was only just buoyant could well have broken away from the wreckage on the way to the bottom, and its slow rate of ascent could account for its arrival back on the surface at a later time. Needs to be explored further.
mm43