That's how you get the dead zone off the shore of Louisiana. The algae doesn't get to the bottom before it starts decomposing, and during calm periods (especially the summer months) the deep waters don't get mixed and replenished with oxygen faster than the bacteria decompose the algae.
You'd need lots of careful planning to avoid causing more harm than good.
The point is to do this in the marine deserts where it is already mostly a dead zone
There are multiple kinds of dead zones. The Gulf dead zones are caused by a lack of oxygen. Iron fertilization, afaik, works in a dead zone that lacks nutrients. A hypothesis for why the dead zones exist is whaling. What poop fertilized the ocean waters, boosting plankton populations, which made whale food. This observed cycle leads to the conjecture that iron is a necessary core nutrient, but you need other nutrients found in whale poop, leading to the creation of synthetic whale poop.
https://www.vice.com/en/article/scientists-are-dumping-fake-...
We do have evidence of iron fertilization boosting algae growth from the Australian fires' iron-rich smoke.
https://nicholas.duke.edu/news/australian-wildfires-triggere...
The big but is whether the iron-based fertilization will grow the right algae or not. Whatever works, we need boatloads of this stuff stat