What is a "safe water" mark?

A "safe water" mark is represented by a white buoy that indicates that there is navigable water in all directions. This type of buoy typically has red and white vertical stripes and can also be equipped with a topmark in the shape of a sphere. The purpose of a safe water mark is to denote that vessels can safely navigate in the surrounding area without the immediate concern of obstacles or hazards in the water.

In contrast, the other options represent different navigational aids or hazards. Markers indicating hazardous areas won't provide navigable channels, while the red buoy mentioned refers to specific maritime interpretations of areas where navigation is restricted due to shallow water. The green buoy marking the port side of a channel suggests a specific course direction for mariners, rather than indicating general navigable water. Thus, the key function of a safe water mark lies in its indication that no obstructions are present, facilitating safe passage.