An acoustic communication system is disclosed as a secondary means for command control of underwater operations from a surface unit. The system includes a diving acoustic command control unit for use in the event of prime propagation path occlusion. Self-check verification as well as data read-out upon interrogation is provided from the surface unit to an underwater receiver-transponder. The latter will also respond to acoustic command signals from the diving unit. An early warning blowout detector employs sensors at the wellhead to detect seismic waves, mud pressure wave fronts and mud flow rates resulting from an emerging blowout and in response thereto, transmits an alert signal.