What is the airspace that extends 3 nautical miles outward from the coast, where activity may be hazardous to nonparticipating aircraft?

The correct understanding for the airspace that extends 3 nautical miles outward from the coast where activity may be hazardous to nonparticipating aircraft is indeed a warning area. Warning areas are specifically designated regions off the coasts of the United States where there may be activities such as military exercises, live-fire training, or other operations that could pose a danger to aircraft that are not part of those activities.

The definition of a warning area indicates that it is meant to alert pilots of potential hazards rather than restricting access, as it is mainly used for information and advisory purposes. Unlike restricted areas and prohibited areas, entering a warning area is not prohibited; however, pilots are advised to exercise caution and remain aware of the current activity status within that area.

The other airspace classifications, such as restricted and prohibited areas, have more stringent restrictions regarding access and are managed to protect certain activities that are deemed more dangerous and less predictable than those found in a warning area. Class B airspace pertains to controlled airspace around busy airports and is not directly associated with hazards posed by activities such as those in warning areas.