That's how you bring several kW of power down to an ROV.
An umbilical cable, copper conductors and fiber optics inside steel wire sheath. The weight of the thing makes it impossible to attach it directly to the ROV - It would be unmaneuverable - so instead they bring it down to a TMS (Tether Management System) - basically a cage where the steel umbilical terminates and a spool of lightweight tether cable, 100m or so, which the crew then pays out to give the ROV the capability to snoop around at depth.
I went to sleep, time zones being what they are, but see that user jacquesm posted a link to a most impressive rabbit hole of engineering porn.
As an aside, one of the first field jobs I attended after graduating was commissioning some equipment fitted onto the R/V Knorr, the very ship which was used for finding the Titanic.
For a Titanic buff, that was a great experience - doubly so when the project manager at the Woods Hole end found out that I had more than a passing interest in the wreck, and during lunch one day suggested we go see if we could find Bob, as he believed him to be on the premises.
Bob, of course, turned out to be Robert Ballard, who turned out to be an incredibly gentle and patient guy who answered each and every question I had like it was the first time he had it posed to him, despite probably having had them thousands of times before...
I've done some work for a company in this domain, it is incredibly interesting and you could easily lose several lifetimes on all of the engineering details.
Leaving aside the literal tons of weight a tether would require just to life a single person back to the surface (and the weight of the tether), it also could get pushed around a lot by the current. And, most importantly, once you are down there and want to navigate the wreck it would get in the way.
The weight of the tether doesn't matter when it is in water. You just design it to be neutrally buoyant. Either using mini pressure vessels or tanks of light oil.
The compressibility of water is fairly negligible - the density increase is only 1% for every 2 kilometers of depth. And obviously the cable also compresses at depth - probably by a similar percentage,
The metal wires in the tether is heavier than water. This is balanced by twining air/gas hoses into the tether. Gas compresses by 50% for each 10 metres of depth in water, so that's a whole different scale.
You either put floats (ie. Gas chambers of fixed volume) or sealed tanks of a light liquid, for example kerosene.
For high depth like this, tanks of a light liquid are probably most suitable. I'd probably use liquid butane. With a vapour pressure of 40 psi, it is easy to contain, and is has a density of only half of water. If it leaks, there won't be an environmental cleanup cost either.
