An open-ocean fish-growing platform has a submersible cage structure for growing fish, an antenna for receiving positioning signals transmitted from an external source, a position-correction apparatus for calculating a position error signal from a target geostationary position, and an ocean thermal energy conversion (OTEC) system for generating electric power for thruster units to maintain the cage structure in the target geostationary position. The OTEC system inducts colder ocean water from a deeper ocean depth for driving its heat exchange cycle, and is also of hybrid type using a fuel-fired unit as a heat source. The cold water effluent from the OTEC system is directed into the cage for flushing wastes generated by the growing fish. The self-positioning, self-powered open-ocean platform enables unmanned, extended marine deployment in deeper ocean waters without the need for tethering or anchoring to the ocean floor.