Dave M
Be careful to use line that doesn't have much of any stretch to it, especially if you're going to splice metal clips into the end.
Line that stretches will store an awful lot of energy, and if one of the clips comes loose or the cleat it's tied to fails, it will "snap back" at the boat on the other end (think rubber band with a paper clip tied to it). I've read of people being seriusly injured that way.
I've read that the pros (e.g., Sea Tow) use special line for towing.
Silver Duck