If growing in the ocean is to be done, it's probably better to try increase the amount of food algae/plankton at the bottom of the chain.

Last i heard, there was some experiment in this area, where a "sterile" part of the ocean was studied, and found that iron is the limiting micro-nutrient. By spreading iron (or rust i guess?) in these dead areas of the ocean, it would trigger growth, which would create a lot more ocean life where none existed before.

But the long term consequences of doing this is unknown. And that's not to mention how to actually get the requisite amount of iron to those places.

Changing an ecosystem is always a risky thing to do though, because there are long food chains, different species living in symbiosis, and just so very many variables.

We will have to get good at it then— we are already doing, just not trying to do it well. Engineering can function even in complex environments.