Fish, yes, but it doesn't help with global warming. The problem is that while you sequester a lot of carbon you end up converting some of the carbon from CO2 to CH4--and that's a much stronger greenhouse gas.
Could you by any chance point me towards any solid quantitative studies of that? The Wikipedia page mentions it in passing, but the reference is pretty unsatisfying:
https://en.wikipedia.org/wiki/Ocean_fertilization