Which channel is generally not a working channel?

Channel 70 is commonly designated as the distress and safety channel for maritime communication. It is primarily utilized for digital selective calling (DSC) and is not intended for routine working communications, which differentiates it from the other channels listed. Channels like 68, 69, and 72 are often used for vessel-to-vessel and vessel-to-shore communications, making them working channels where more frequent interaction and operational exchanges can take place.

Since Channel 70 is reserved specifically for emergencies and safety-related transmissions, it is understood that it is not part of the working channels used for day-to-day operations among vessels. This unique purpose clearly identifies Channel 70 as the answer in the context of the question.