What signal could be used by a shore rescue unit to indicate "landing here is highly dangerous" during the daytime?

The indication for "landing here is highly dangerous" during the daytime is represented by horizontal motion with a white flag. This action serves as a universal signal recognized by mariners, indicating specific safety instructions. The horizontal waving motion conveys urgency and alerts individuals that conditions are not safe for landing.

This method of signaling is effective because it utilizes visual cues in a way that can be easily seen from a distance, especially for those at sea. The motion of the flag draws attention and clearly communicates the potential danger associated with landing at that location.

When considering the other options, while a red flag with a white cross is a recognized signal for other safety warnings, it does not specifically denote danger in landing. A series of loud whistles might alert a nearby vessel but lacks clarity compared to a visible flag signal. A yellow flag raised high typically signifies caution rather than the explicit warning of danger in landing. Therefore, the most appropriate and understood signal for communicating that a landing is dangerous is the horizontal motion with a white flag.