The correct answer is C) underwater. When towing astern in the ocean, keeping the towing hawser (the line connecting the towing vessel to the towed vessel) underwater helps reduce stress on the hawser. An underwater hawser experiences less wave action and surface currents, which can create tension and strain on the line. Keeping the hawser underwater helps dampen these external forces, minimizing the risk of the hawser parting or the tow being disrupted. The other options are incorrect because: A) just touching the water still leaves the hawser exposed to surface forces, B) making the hawser as short as possible may not be practical or desirable in all situations, and D) securing the hawser to the aftermost fitting on the towing vessel does not address the need to submerge the line.