Which of the following is a sign of a potential hazardous condition on the water?

The choice indicating that dark clouds gathering rapidly is a sign of a potential hazardous condition on the water is correct. This situation can indicate an approaching storm or severe weather, which poses risks for boaters, including high winds, heavy rain, and lightning. Rapidly changing weather can lead to dangerous conditions that make navigation challenging or unsafe.

In contrast, calm and sunny weather typically suggests stable conditions that are safe for boating. An increase in water bird activity can indicate various factors but doesn’t inherently signify danger. Clear visibility of distant boats also implies safe conditions, as it suggests a lack of obstruction and good weather. Therefore, recognizing dark clouds as a warning sign is crucial for ensuring safety while on the water.