The problem with the underwater plumes is that they can't be contained. While you may have the image of an underwater oilslick, it isn't - the oil can be measured in parts per million (you might not even notice if you had a glass of it).

The problem with those plumes is that bacteria eats the oil, and then breeds. Those organisms use up the oxygen in the water, leaving a dead zone unfit for life. There was already a decent sized dead zone from the Mississippi, but this has the potential to dwarf it.