[eru]

You are right that we are past what hunter-gatherer approaches can yield from the ocean. And that we are doing a bad job at protecting the habitat of wild-life.

However, there's vast patches out in the open ocean where almost nothing grows. Mostly because the areas with sunlight (at the top) are not where the minerals are that plants need to grow (mostly the sea floor). See eg https://en.wikipedia.org/wiki/Iron_fertilization for an overview.

If you mind putting fertilizer directly into the open ocean, someone more clever than me can probably work out a scheme for enclosing some water in a giant floating bag, and growing your stuff in there. (Or something much better than this.)