A navigation system extends satellite navigation to divers. The navigation system comprises a surface unit and a plurality of sub-surface beacon units. The surface unit includes a receiver to receive navigation signals from earth-orbiting satellites, processing circuits to communicate with to sub-surface beacon units and to transmit location information to said sub-surface beacon units, and a sonar transmitter to transmit location information to the sub-surface beacon units. The beacon units include a processing circuits to determine the location of the beacon unit based on location information received from the surface unit, and a sonar transceiver to receive location information from the surface unit while the surface unit is floating on the surface; and to transmit location information to a diver unit to provide navigation assistance to the diver unit.