A system for the transmission of signals to or between underwater installations, uses optical fibres for the transmission of optical signals to/from a control unit or between the installations. The system includes a conversion unit coupled to one of the ends of the optical fibre for conversion of the optical signals to high frequency radio signals and/or conversion of high frequency radio signals to optical signals, a conversion unit having a coupling to an electrical power supply cable for the transmission of power to the underwater installation, where the coupling unit is coupled to the conversion unit to transfer the high frequency radio signal along the power supply cable to and/or from the underwater installation, and at least one transducer unit in the underwater installation for receiving and/or emission of the high frequency radio signal.