Need to be very careful when towing another boat with a normal boat - you can easily fry your engine if going much more than headway speed - 5mph, due to the extreme engine load. (Did that once long ago, the 3-cyl Merc 65 was never the same after that.) The damage may not appear be immediately, but will show up as shortened engine life. Towboats have huge engines and large short-pitch props, and probably other modifications so they can sustain a lot of tow power at low speeds.
[4-cycle]: If you must tow another boat, consider changing your oil afterwards, or at least check it for a burnt smell or bad color.
In defense of using the ski line: It floats, so easier to keep out of your prop, and it is weak enough that it will break if you tow too hard.
