What type of flag indicates a diver is in the water?

A red flag with a white diagonal stripe is the standard maritime flag used to signal that there are divers in the water. This flag serves to alert boaters to exercise caution in the area, as there are individuals submerged and potentially swimming to the surface. The red color indicates a warning or danger, while the white diagonal stripe enhances visibility and distinguishes this flag from others.

The blue flag is often associated with scuba diving, particularly when used in specific contexts, but it is not the universally recognized signal for a diver in the water. The green flag typically indicates that it is safe to navigate through the area, and the yellow flag is often used to signify caution or a general warning but does not specifically imply the presence of divers. Thus, the red flag with a white diagonal stripe is the most appropriate and recognized signal for indicating that divers are present in the water, ensuring safety for both the divers and other watercraft.