Mike,
Probably the 'quickest' way of doing that is by towing a floating line and 'hook' behind the boat. Circle the derelict so that the line and floating 'hook' get caught on the vessel's prop/rudder/whatever. When the vessel is 'hooked up' pull it to where it needs to go. That's probably the 'quickest' in terms of making the gear you'll need. It isn't always easy or simple in practice.
How long of a line is needed? Sort of depends on how big the derelic vessel is. Big boats require long lines, small ones shorter lines. How about the floating 'hook'? How about a chunk of foam with three or four brass wire 'hooks' sticking out of it, sort of angled back toward the tow line? They catch on anything sticking out of the derelict, or the tow line after making a couple pf passes around the derelict. (Or high speed pass with really sharp hooks that'll penetrate the hull?? Probably get disqualified though.)
I'm sure there are a lot of suggestions/ways of doing it. Picking the 'right' one for you is the 'trick'. You might take a look here;
http://www.vac-u-boat.com/Phil has done some research about towing for his boats. Not all of them will apply, but who knows?
- 'Doc