If you carry a VHF radio onboard (and you should), you must maintain a watch on channel 16 when the radio is on and not being used to communicate. You may also maintain a watch on VHF channel 9. Note that urgent marine information broadcasts, such as storm warnings, are announced on channel 9 only in USCG First District waters (northern New Jersey, New York and New England).
Most radios have a memory scan option where you can add specific channels to the memory and press scan. The radio quickly switches through and listens to each channel, pausing if someone is using that channel and then resuming the scan.