What should never be relied upon for exact positioning?

The correct answer is that a floating aid or buoy should never be relied upon for exact positioning. While buoys are useful for general navigational guidance and provide information about nearby hazards, they can drift due to currents or wind. This variability can make them unreliable as fixed points for precise location. The position of a buoy may not reflect the actual charted location, especially if environmental conditions have caused it to shift.

In contrast, charts are designed to show the precise locations of navigational aids and underwater features, while GPS devices provide real-time, highly accurate positioning suitable for navigation. VHF radios, although primarily used for communication, can also be part of a position-fixing system when combined with other tools or when receiving navigational updates. Therefore, while all the other options offer valuable information for navigation, only floating aids lack the reliability needed for exact positional references.