Exactly what do you think the bathyscaphe was, if not a steerable submersible? It was not connected to a surface ship, carried it's own power for steerable thrusters, and dive time was limited only by battery power and crew endurance. After a crack developed 5 hours into the dive they continued to the bottom, took pictures, and returned safely to the surface.
Regards,
GtG
PS They reported seeing fish at the bottom of the trench!
I wasn’t aware that you could steer it, so thanks. That is interesting.