In the worst-case scenario, underwater carbon sequestration wells could fail and release the stored carbon, killing off the plants, fish and even the people in boats in the waters above. Storing carbon also has potential global implications, if, as opponents claim, carbon capture and sequestration will allow the fossil fuel industry to maintain the status quo as one of the world’s top emitters of greenhouse gasses.
Since this is almost certainly going to be what fossil fuel and other ultracapitalists want, I would expect lots of corner-cutting that leads to failed wells, followed by some toothless wrist-slapping by whatever remains of the regulatory bodies tasked with oversight of these companies.