Explanation: Ch-70 is the dedicated Digital Selective Calling (DSC) watch channel for VHF marine radio. All DSC-equipped vessels are required by regulation to continuously monitor Ch-70 for distress alerts, urgency calls, safety calls, and routine call requests. This digital channel allows for automated distress alerting and efficient establishment of voice communications on a working channel. Ch-16 is the traditional international voice distress, safety, and calling channel, which is also vitally important and often monitored. However, modern maritime radio systems (GMDSS) mandate continuous, automated digital monitoring of Ch-70 for DSC alerts. Ch-28 and Ch-80 are typically working channels used for routine communications or port operations, not designated as primary continuous monitoring channels.