Think about all the methods used throughout human history to determine location. Looking toward distant mountain ranges, determining the positions of the stars,

Think about all the methods used throughout human history to determine location. Looking toward distant mountain ranges, determining the positions of the stars, measuring shadows cast by the sun, receiving signals from GPS satellites... none of those work underwater. The support vessel relies on acoustic pings to track the submersible. If the submersible stops responding, or is out of detection range, you'd have no idea where it is. "Rigidly planned route" is not really the case either. You're not going to drop in at the exact same GPS coordinate, and there might be currents that push you.