OK, assuming the premise that microbes that are “eating” the oil and gas are depleting isolated areas of the ocean of oxygen is true, I have an idea. Sure, this may sound like it belongs on half-baked.com but here goes. Why not drop electrodes down into these submerged plumes and run a current though them. This would create oxygen bubbles that could be absorbed back into the water and replenish the depleted supply. Essentially, we would be using the energy from the electricity, through electrolysis, to supply organic life with the fuel it needs to break down the oil.
Two things —
(1) the damage is NOT irreparable, but it’ll take a variable ‘while’ for nature to right itself. So if we do nothing about these dead zones (MOST ALREADY THERE FROM FERTILIZER RUNOFF) caused by the spill, nature will work it out over time. Keep in mind that many areas of the ocean naturally have very low levels of O2, and others do not. The problem in the Gulf is that areas that should have O2 are losing it.
(2) In theory your idea is sound, but in practice you’d have a hard time getting enough O2 dissolved in the right places. But a clever idea. IT’d be like spitting into the whirlwind. It might be easier to add chemical oxygenation or just send big hoses below for ‘bubblers’. But the idea of physically augmenting the levels of O2 is a good one in theory, but the scale needed would make it hard in practice.
Sorry, but in salt water, it doesn't work that way. Instead of oxygen, you get chlorine.