Bill, I recently put up a video that addresses these issues but I will summarize.
The overall themodynamic efficiency of OTEC is low and you do have to move a great deal of heat to the deep but this is an environmental benefit because on the surface this heat drives tropical storms and sea level rise. The recent warming hiatus is believed to have been caused by the movement of surface heat to a depth of about 100 to 300 meters in the Eastern Pacific due to unusually strong trade winds.
Indications are this will be an El Niño year which could return a good portion of that heat making this year and next record setters.
OTEC as I and a small group of others propose it should be implemented uses a deep water condenser design, which would move "a great deal" of surface heat to a depth of 1000 meters thus making it that much more difficult for this heat to return plus the fact this surface heat would be constantly driven to these depths by large scale OTEC implementation. At 1000 meters the coefficient of expansion of sea water is half that of the tropical surface so you gain a sea level benefit by relocating some of the surface heat.
The deep water condenser also addresses conventional OTEC's cost and environmental issues.
Prof. Gerard Nihous, Department of Ocean and Resources Engineering, University of Hawaii estimates that the maximum steady-state OTEC electrical power is about 14 TW (Terawatts)
The greatest climate risk to biological life in the oceans is thermal stratification. Moving surface heat to deeper water reduces the problem and induces convection that would bring the nutrients phytoplankton require to the surface, which in turn could reduce atmospheric carbon dioxide.
With conventional OTEC there is also a risk of impingement and entrainment of marine life and eutrophication due to bringing too much nutrient rich water to the surface. With the deep water condenser design the only water movement is at the surface over the evaporator. The working fluid has to be pumped back to the surface rather than bringing cold water upward to the condenser. This decreases parasitic losses of the system as you are moving about 1/20th the amount of fluid.
At the surface wave action and disbursement of the evaporator can limit the impact on sea life.
Regarding the thermohaline circulation, Nihous takes this into consideration in his estimate of 14 terawatts. I also have considered a counter-current heat flow system that could lessen this impact even more.
The lesson from Nature the past fifteen years would seem to indicate that sea level rise would in fact be reduced by moving heat to deep water. When this heat drives tropical storms, it is fast tracked to the poles where it melts ice and more ominously permafrost.
As to cloud formation, low-level clouds reflect sunlight and lead to cooling, high level clouds warm the planet increasing the the greenhouse effect. Recent studies indicate that as the ocean warms, low-level clouds tend to dissipate creating a positive feedback warming cycle.
To the best of my knowledge there is currently only one OTEC plant operating at a capacity on only 50 MW. I think however this is more a reflection on the cost and environmental problems of the conventional design, rather than the one my colleagues and I are trying to promote.