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.
I didn't interpret that we were talking about fire extinguishers or O2 tanks.
I once took an 8 oz styrafoam coffee cup and wrote some words of prose on it and sent it down 15000 ft on a dive. When it came back up still attached to the submersible it was the size of a perfect sewing thimble with words that had to be read with a magnifying glass. Of course it would sink like a rock after that.
Of course any thing with the scale factor of a 4 inch thick titanium 8 ft dia sphere would survive. As far as hover rates at part depths, that's another story as long as you have an idea of the various depth currents in a computer plot over several months