The tow line should be secured to the vessel being towed, then the tow vessel should adjust the length of the line equal to the wavelength (or two or three wavelengths). That way both vessels are climbing and surfing the waves simultaineously. This lessens the shock/slack cycle every time you crest a wave. I have done this when pulling a power boat in 4' seas with my power boat. It may take some time to start and stop every time you make an adjustment but it is easier on the equipment when you get it right.