[whats_a_quasar]

The Jason ROV has a six mile tether:

https://en.wikipedia.org/wiki/Jason_(ROV)

And the Titanic was discovered by camera sled being towed by a ship.

https://en.wikipedia.org/wiki/Argo_(ROV)

So your search might have been a bit too quick. It certainly is possible possible to build a tethered crewed submersible, though this vehicle wasn't.

Credit to js2 from this comment: https://news.ycombinator.com/item?id=36396161