When must a commercial vessel display its registration number?

A commercial vessel must display its registration number always while on the water to ensure compliance with legal regulations and safety protocols. This requirement facilitates the identification of the vessel by authorities, enhances accountability, and aids in the enforcement of maritime laws. Displaying the registration number helps prevent unauthorized use of the vessel and ensures that it is easily recognized during patrols or in case of emergencies.

In contrast, the other options suggest scenarios where the registration number might not be displayed, which would breach regulatory requirements and compromise safety. For example, only displaying it when docked or during competitions does not meet the continuous visibility requirement mandated for commercial vessels, and hiding the registration number is contrary to regulations that aim to ensure the transparency and accountability of all operational vessels.