A spherical buoy, resembling a ball, is known as what?

A spherical buoy that resembles a ball is referred to as a safe-water mark. This type of buoy is specifically designed to indicate that there is safe water nearby, typically marking fairways or channels. The shape and color of a safe-water mark are standardized and are often red and white striped, helping mariners identify their position and navigate safely.

Safe-water marks serve an important role in maritime navigation, providing visual reference points that indicate open and navigable waters. By signaling the presence of a safe-water area, these buoys help prevent vessels from running aground or encountering hazards in the water.

The other options represent different types of navigational aids: an isolated danger mark indicates a specific hazard that has navigable water all around it; a regulatory mark provides information related to rules or regulations, such as speed limits or no-wake zones; and a daymark is a stationary structure or buoy painted with colors or patterns to aid in navigation, typically only visible during the day. Since the question specifically describes a spherical buoy, the safe-water mark is the correct identification.