Well, it turns out I'm starting a tugboat project and have been doing some research, so here goes ...
It depended on what the tow was.
Towlines were often wire cables with a rope section in the center to act as a spring. They were held together with swivels, clevises, bowlines, or a combination depending on the boat and the tow. Sometimes the spring was not used.
At the towed end the wire went over the casualties' bow or stern as appropriate with chain and/or bridles to prevent chafing the wire. If the wire went aboard the casualty then it was tied to the ship's bitts and a sharp eye and a lot of grease were applied where the wire left the ship. Often the casualties' anchor chain was fed out for this purpose, in which case the towing wire was shackled to the chain with a swivel.
In most of these cases the actual wire would end in an eye splice around a thimble, which would be frapped with more wire.
Hope this helps,
Bruce