In addition to a displayed diver-down flag, what indicates that a diver may be below the water's surface?

A diver's bubbles are a key indication that a diver is below the water's surface. When a diver is submerged, the air they exhale forms bubbles that rise to the surface. Observing bubbles emerging from the water acts as a visual warning to nearby boaters and swimmers that there is likely a diver beneath, thus emphasizing the need for caution in the area. It is an important safety measure to ensure those in the vicinity are aware of the presence of divers below the surface.

While dive equipment on the surface can suggest that a diver is underwater, it is not as immediate or conclusive as the presence of bubbles. A safety buoy can indicate specific diving areas and alert boaters to be cautious, but it does not provide direct evidence that a diver is currently beneath the water. A flag with a 'D' symbol is not standard; the recognized signal for divers is the diver-down flag, which is a red flag with a white diagonal stripe. Therefore, diver's bubbles serve as the most reliable, immediate indication of a diver's presence below the water.