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