Who constantly monitors Channel 16?

Channel 16 is designated as the international distress, safety, and calling frequency in the VHF marine radio band. The Coast Guard is responsible for constantly monitoring this channel to ensure that they can promptly respond to emergency calls. This monitoring allows the Coast Guard to receive distress signals from vessels in trouble, coordinate search and rescue operations, and provide assistance to boaters in emergency situations. This critical role underscores the importance of Channel 16 as a lifeline for mariners, particularly in emergencies where timely communication can save lives. The other groups mentioned do not have the same level of responsibility or capacity to monitor this channel continuously, which is why the Coast Guard is the correct answer.