OTEC
It's a valid concept, and operates like any other heat engine by taking heat from someplace warm and dumping it someplace colder, while extracting energy. As with all heat engines, the size of the differential has a lot to do with efficiency, and the differential between the ocean surface and deep sea is not that great compared to, say, a steam engine. There are some technology issues. And it can't compete economically with conventional energy sources except for certain geographical areas (tropical islands, for example.) Still, with a working media that has a low boiling point, it can work.