If you encounter a vessel that has only a white light showing, what does this indicate?

If a vessel has only a white light showing, it indicates that the vessel is at anchor. This is signified by the presence of a single white light, which is required by navigation rules when a vessel is not underway. This light serves to alert other vessels of its presence to avoid collisions.

In the context of the other options, if you were overtaking a vessel, you would typically see combination lights such as a green or red light, along with a white navigation light. The information regarding vessels transmitting a distress signal usually involves signaling devices or specific light patterns, such as alternating colors, rather than a single white light. Lastly, a sailing craft would display specific lights, including a red and green combination, along with a white masthead light when underway. Thus, the specific situation described in the question pertains directly to an anchored vessel, making it important for mariners to recognize this signal to navigate safely.