[SoftTalker]

It's dependant on a support ship but it's not tethered to it? Maybe depth prohibits that?

[bagels]

Yes, this was my thought too. Titanic depth, I think, would not be too deep for steel cables to reel it back in, but marginally. It could also double as a communication link.

[the_gipsy]

There have been tethered submersions deeper than the titanic wreck.