Explanation: VHF-FM Channel 13 is specifically designated by FCC regulations for mandatory Bridge-to-Bridge watch by commercial vessels and vessels 20 meters or more in length in most areas of the continental U.S. This regulation ensures a dedicated voice channel for collision avoidance and safe navigation communication between vessels, especially in busy waterways. Channel 13 is often restricted to 1 watt power to limit its range and localize communications, preventing interference over large distances. DSC on Ch-70 (A) is for digital selective calling, primarily for distress alerting and making initial contact, not for continuous voice watch. VHF-FM Ch-16 (C) is the international distress, safety, and calling channel, requiring vessels to shift to a working channel for routine conversations after initial contact. A vessel's VHF working frequency (D) is for general communication once contact is established, not a universally designated watch channel.