Like so many idiomatic expressions, “toe the line” has nautical origins. It meant to put your toes on a plank or line drawn on the deck, usually for disciplinary purposes. In other words, “toe the line” means to show willing conformity with the requirements of an authority.
Also, there are many lines on a boat, but only a few ropes, and the tow rope is one of them. No well-drilled sailor would ever call a tow rope a “tow line”, unless he wanted to meet the captain’s daughter!