What distinguishes being “underway” from being “aground”?

Answer: C — Not being anchored, grounded, or attached to shore

The distinction between being "underway" and being "aground" primarily revolves around the vessel's relationship to the water and its position relative to land. When a vessel is classified as "underway," it means that it is not anchored, aground, or attached to the shore, allowing it the capability to navigate freely through the water. This definition captures the essence of being in motion or capable of movement. In contrast, a vessel that is "aground" is stuck or grounded on the bottom, preventing any movement through the water. Thus, option C effectively encapsulates the concept of what it means to be underway, emphasizing that the vessel’s status is defined by its lack of attachment to any fixed point, such as the shore, which is crucial for its operation and navigation. This definition aligns with the COLREGs as it helps to establish navigation rules and responsibilities while a vessel is in motion.