What is a visual indication that divers might be below the water's surface?

A diver's bubbles are a key visual cue indicating that divers are submerged beneath the water's surface. When divers exhale, the air they release forms bubbles that ascend to the surface. This serves as a clear and immediate sign to boaters and others on the water that there may be individuals diving below. Recognizing these bubbles is crucial for safety, as it alerts operators to exercise caution and ensure they maintain a safe distance from the diving area.

The other options, while relevant to diving, do not serve as direct visual indicators of divers being below the surface. A diver-down flag signals the presence of divers but does not directly visualize them, dive lights may indicate underwater activity but are not exclusive visual indicators of divers, and a signal buoy suggests a designated area but may not always indicate the immediate presence of divers. Therefore, bubbles provide the most direct and clear visual indication of divers in the water.