What event requires the use of a diver’s flag?

The use of a diver's flag is specifically required during diving activities to ensure safety and visibility. This flag serves as a warning to other boaters in the area that there are divers submerged, thereby reducing the risk of accidents. The diver’s flag is typically a red flag with a white diagonal stripe, and displaying it alerts nearby vessels to maintain a safe distance and avoid the area where divers are active.

In the context of water skiing, fishing, and sailing, these activities do not necessitate the same level of caution regarding the presence of submerged individuals. Water skiers are generally visible on the surface, and fishing and sailing do not involve the kind of submerged activities that warrant a diver’s flag. Therefore, diving activities are the only situation listed that rightly necessitates the use of a diver's flag for safety reasons.