The April 2009 issue of Sea Kayaker mag gives both an example situation where a tow-ee needs to release and the tow-er can't, and the solution in the form of a fast, simple, quick release knot for the tow-ee end of things.
The tow-er gives the end of the rope to the tow-ee, then ties the Highwayman's Hitch knot (which doesn't require a rope end to complete) to the deck line. The tow-ee can pull the rope to release the knot. No 'biner involved.
The knot is also useful for cartopping, that way you get lots of practice using it.