USCG Navigation Center guidance says channel 16 is for distress, safety and calling. When a routine call is answered, both stations should move promptly to an appropriate working channel.
In general, a vessel equipped with marine VHF must maintain the required watch when the radio is not being used for another communication. Larger vessels can have additional channel 13 obligations.
Know the channel table, keep transmissions brief and never use channel 70 for voice; it is reserved for DSC.