A vessel is "in sight" of another vessel when __________.

The definition of a vessel being "in sight" of another vessel specifically refers to the ability to visually observe one vessel from the other. This concept is crucial in navigation and collision avoidance, as it helps determine the responsibilities of vessel operators under the navigational rules. When vessels are in sight of one another, they can take appropriate actions to avoid collisions and ensure safe passage.

The other options, while related to navigational situations, do not accurately capture the definition of "in sight." Simply being in the same waterway does not guarantee visual observation; vessels may be nearby yet not visible due to obstructions or distance. Having a radar operational does not define visual contact—it simply means that equipment is ready to assist in navigation. Lastly, the presence of light does not necessarily imply that vessels can see each other; visual sighting depends on more specific conditions such as distance, angle, and visibility. Therefore, option A best represents the correct understanding of what it means for one vessel to be in sight of another.