You need to take away the energy used by the devise itself in moving huge quantities of water huge distances.
This is what the heat pipe does, it moves huge quantities of heat through phase changes of the working fluid.
With conventional OTEC you move 400cubic meters/second of water to produce 50 megawatts. With a heat pipe you have to move 8 cubic meters of liquid ammonia back from the condensing end to get the same result.
As Paul Curto, former Chief Technologist with NASA points out with this design, "The parasitic losses are cut in half. The costs for the cold water pipe are eliminated, along with the cold water return pipe and condenser pumps, the cleaning system for the condenser, and the overall plant efficiency approaches 85% of Carnot vs. about 70% with a cold water pipe.
The parasitic losses could be reduced as much as 50% and the complexity, mass (and cost) of the system reduced by at least 30%. The vast reduction in operating costs and environmental impacts would be worth investigation alone.