There are already many aquaculture pens in the ocean near Port Lincoln, South Australia, often for tuna. You see them from the air when flying over. Here's a photo of what they look like:
The issue is that many of the desirable farmed fish like tuna and salmon are carnivores. Farming them still requires catching mass quantities of less-commercially-useful fish along with the issues that has.
Edit: I should say part of the issue. There are other ecological concerns with farmed fish.
https://www.cleanseas.com.au/custom/files/media/our-farm-2.j...