I'm used to working with things like this...a work class Remote Operated Vehicle (ROV).
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They work down to about 3000 m and operate on a tether, a wire to lower them up and down on, and the tether includes an umbilical that is used to communicate with it and send commands. They are pretty high tech...a friend that works with the biggest manufacturer also led their bid for the next NASA space suite a few years ago. Typically they will have an acoustic beacon, so you can monitor their position from the surface, and a sonar so they can find things. The sonar images are pretty basic, shall we say. They certainly are not getting a picture that's easy to interpret, so you steer them using camera images.
Tether and umbilical management is a skilled business. When the ROV is working you need it to swim about unimpeded, but the cable floating around can get into a tangle, and caught up on stuff.
The manipulator arms are pretty powerful, and you can fit various skids with things like hydraulic power packs. They are not very powerful when it comes to pushing and pulling stuff, and it would be a struggle to see how they could deliver buoyancy to a dead submersible...in the oil business that is typically done by crane, lowering buoys on counterweights, then releasing the weights. The ROV hooks the buoyancy onto what you want to lift.
The sub that's lost I don't really understand, and I'm on a ship at the moment and they block YouTube.
I assume it has a very light umbilical to communicate between the surface and their playstation controller. I assume there is no tether, so they can't hoist it back to the surface. I assume they have some variable ballast tanks, and compressed air to maintain their altitude, and some small propellers to manoeuvre with. I assume there are solid ballast drop weights that are simple to dump to get positive buoyancy to come to the surface.
What seems strange to me, reading the press reports, is people taking about radio...radio waves really do not work well under water, and the fact that previous tourists talk about losing communications a lot. Losing comms with an ROV is rare, and you would immediately think about sending another one to find out what's going on. Maybe they became blasé about short interruptions to comms and either end assumed all was well when it wasn't.
The mother ship is ancient. It was built in 1959 as a lighthouse tender. It is not dynamically positioned. It really doesn't not look equipped to deal with much in the way of emergency recovery. From the photos, I can't see that it is equipped with a large ROV, and if it can't do stationkeeping, it is not a very useful resource.
I assume oil industry support vessels with ROVs will be going in that direction from the Canadian oilfield...but at 3,800m water depth the ROVs may not be able to operate in the water depth.
I imagine the friendly navies with nuclear submarines will be sending boats in that direction too...they may be able to detect sounds inside the submersible, and that may be able to use active sonar...but finding a tiny bit of tube in the middle of the Titanic debris field with systems designed to detect other submarines may be tricky. I suspect the navies may not wish to confirm or deny submarines in the area.
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