why is it called "Guard"?
One of the stated missions of the US Coast Guard is,
Maritime Safety: Eliminate deaths, injuries, and property damage associated with maritime transportation, fishing, and recreational boating. The Coast Guard's motto is Semper Paratus—(Always Ready), and the service is always ready to respond to calls for help at sea.
Taking a punt, would it date to the maritime days and mariners referring to monitoring "Guard"? ie Coast Guard.