Paul, Gerard Nihous, Hawaii Natural Energy Institute, is considered the expert on this subject. Five years ago he raised his estimated maximum steady-state OTEC figure from 3 to 5 terra watts. As I subscribe more to the Richard Smalley school that says we may need as much as 60 terrawatts (perhaps as much as half of this for desalination) I tried to think of a way to maximize OTEC's potential and came up with a counter-current heat transfer system that captures the latent heat of condensation of the working fluid and returns it to the surface. The analogy is the same way rain from a hurricane returns much of the 50 to 200 terrawatts of heat that a major storm produces back to the surface. As there are as many as 21 major storms a year globally I see no reason why OTEC cannot provide all of the power we need by converting the electricity produced to the energy currencies hydrogen, ammonia or methanol.
Susequently Dr. Nihous has upped his estimate to as high as 25 TW even without counter-current flow.
The problem is the more heat you dump to the depths the less efficient OTEC becomes and the less power potential you have. A heat pipe can dump a lot of heat rapidly and enough of them could alleivate a situation as we have had in the Atlantic recently but probably it would be best to try and convert as much of this heat to power as possible.
Another interesting thing is the colder water gets the less it expands. Below 4C is actually contracts, so dumping ocean heat to the depths where it is diluted would have a minimial positive impact on sea level rise.
The greater sea level impact is derived from reducing thermal expansion and converting liquid volume to gas to produce hydrogen and oxygen which can benefit ocean dead zones.
As to the obstacles Robert Cohn blames a strong nuclear energy lobby http://www.marineenergytimes.com/part-iii--could-china-be-interested-in-taking-the-lead-in-developing-the-technology.html