I can understand your frustration piongain ... your original thought seems to have been whether or not floating wave-energy plants could generate enough energy to sequester enough carbon to make a significant impact on CO2 in the atmosphere.
I think this has been answered as a decisive no. Wave energy plants (such as Salters' duck) are tethered - a floating vessel could theoretically capture wave energy, but at a much lesser efficiency. And total wave power capacity (according to Wikipedia) is only 2TW.
The energy needed to sequester a significant amount of carbon is vast, as sidd pointed out it's in the region of 1e22J per trillion tons =~ 100 years of man-made CO2 release at current rate. To keep up with emission, 1e20J would bee needed every year.
You also asked about harvesting energy from temperature differentials, i.e. OTEC.
OTEC has potential capacity of 88.000 TWh/yr according to Wikipedia, or ~ 3e24J ... so theoretically yes, OTEC could be used to produce enough energy to sequester manmade CO2 emissions (if my calculations and presumptions hold water) ...
But in reality, when compared to current annual world energy consumption at 5e20J, I don't really see any change of this ever becoming a reality.