[jfengel]

Additional CO2 in the oceans is indeed very bad for marine life. It makes the ocean more acidic. Ocean pH has dropped from 8.15 to 8.05, so far.

Pumping O2 into it isn't really any better. Sea creatures evolved for a certain equilibrium. Adding more of anything will throw off the balance. Some organisms would benefit, but others won't, and so the population levels can shift, throwing off the entire food web.

Those webs are pretty resilient, and it takes an awful lot of emissions to alter them significantly. But we've managed to overcome a huge amount of buffering to increase ocean acidity, and that's potentially very, very dangerous.