The vehicle that discovered the titanic was a sled pulled behind a ship on a tether. A thick steel cable can take a lot of load. https://en.wikipedia.org/wiki/Argo_(ROV)

Also, subs operate at close to neutral buoyancy. A tether does not need to support the weight of the vehicle, it only needs to resist the force of drag that the water exerts on the vehicle moving at whatever the max tow / lift speed is.

The vehicle that