A high efficiency OTEC system for the efficient and low-cost production of energy and potable water through ocean thermal energy conversion. The high efficiency OTEC system utilizes a working fluid such as ammonia with a standard Rankine cycle. One embodiment of the present invention comprises a heat exchanger which includes a fluid transfer assembly, a pump, a first heater, a turbine and a generator. The fluid transfer assembly utilizes a condenser conduit to condense working fluid by transferring the fluid to the cold depths of the ocean before returning to the warm surface through an insulated return conduit. The working fluid is vaporized through use of the first heater before entering a turbine connected to a generator to produce energy. In some embodiments, a second heater may be added to the system for production of potable water. In further embodiments, a third heater may be provided for additional heating capabilities.