It is indeed a pitty. Imagine how much carbon could be captured if these pesky seas had their fair share of trees.
It's quite hard to imagine, because those trees could mess up other carbon sequestration cycles already present in oceans. E.g. phytoplankton uses CO2 but need sunlight to do so.
Or maybe we should replace all the trees by Oceans, because they're a much more efficient carbon sink !
Well good news then! This seems to be the current strategy.