We could do it inside of 10 years.
OTEC? Nope. Too costly, too inefficient. As I recall, OTEC plants have projected efficiencies in the single digits. That makes them HUGE...and any small errors can drive the efficiency to zero or negative. Nobody will invest in such a flawed concept.
--Boris
And if they are huge, that means the only economical OTEC plant would be open-cycle (i.e., it would circulate actual seawater, instead of ammonia). The problem with such systems is that they biofoul in short order. Every few months--or weeks--their innards would need to be scoured of barnacles, mussels, and whatever else can cling and grow.