If this is extracting electrical power, it has to be producing a net drop in thermal energy in the ocean; cooling the warm surface layer slightly more than it is heating the cold sub-surface layer. Energy must be conserved.
But regardless, the problems of global warming are less 'adding heat to the planet', and more 'adding heaters to the planet'. CO2 causes the atmosphere to capture more solar energy continuously, which is a much larger long-term problem than adding some (or even a lot) of heat in the short term.