Ocean thermal energy conversion (OTEC) uses the temperature difference between cooler deep water and warmer surface-level water to run a heat engine to produce useful work, usually in the form of electricity. Fresh water is a byproduct of this process.
There is a lot of debate and worry about climate change. Most are worried about warming and sea level rises, which would cover coastal land and reduce usable landmass. Others are worried about global cooling. A significant swing in either direction would substantially change the latitudes at which human life is most comfortable.