There are several OTEC projects in the works around the world.
According to the 2014 JRC Ocean Energy Report, they estimate that between 30,000 and 90,000 TW/h per year of power would be extractable without having negative impacts on the thermal characteristics of our oceans.
Ocean thermal energy conversion (OTEC) is a technology that captures energy from the temperature difference between cooler deep ocean water and warmer shallow ocean water and uses that energy to produce electricity.
The first OTEC plant was a 50 KW plant than was first operational in 1978. Now there are several plants being designed. See the chart below.
How OTEC Works
Here is a video about OTEC
