What condition could impair a vessel's visibility?

Heavy rain can significantly impair a vessel's visibility for a number of reasons. When heavy rain occurs, it creates a curtain of water that obscures the view of other vessels, navigational markers, and potential hazards in the water. This diminished visibility can lead to difficulties in spotting critical objects, aligning with safe navigation practices, and determining distances. Additionally, rain can make it challenging for mariners to see the horizon or differentiate between water and land.

In contrast, bright sunlight and a clear night typically enhance visibility, as they provide adequate light for observing surroundings. A calm sea, while safe in terms of navigation, does not affect visibility directly. Thus, heavy rain stands out as the condition with the greatest potential to obstruct a vessel's line of sight and create navigational challenges.