Aquaculture has the best ratio, but fish farming isn't the only way to do it.
The GreenWave folks seems to have a decent alternative. They base their aquaculture on filter feeders (oysters, mussels, scallops, clams) and autotrophs (ocean kelp, a salad green which btw is the source of omega-3 in fish), with the result that their farms clean up fertilizer runoff from terrestrial farms and reduce eutrophication. Their protein requirements are even lower than fish farms, requiring 0/7ths of what a cow needs (ie no inputs).
To be fair, chinese do exactly the same since 1950, when they invented and polished the modern polyculture systems based in seashells, ocean kelp, and crabs or fishes (and a similar freshwater version since thousands of years ago). Culture of marine filter feeders is not exactly breaking news for spaniards, french, dutch or japanese. If you can't resist the urge to invest on them, remember that GreenWave is just another company doing the same as thousands of current asian and european companies. Would be not much different than claiming that a californian company invented wine in 2017.
Attribution aside, I'd like to see the technique more widely known and practiced. The commenter was defending fish farming by favorably comparing it to cows, but ignored shellfish polycultures probably because they simply didn't know about them.
It appears to be mainly China that practices it widely. To the extent the Chinese aren't scaling it globally as a climate-friendly alternative to fish farming, they leave an opportunity for other players.
And to come back to the topic of the original post: there's an invasive species of Japanese oysters in the Dutch Wadden Sea that you're free to harvest if you're in the neighbourhood. Although the debate on whether they are an ecological disaster or help restore the environment is not set, it's probably going to be the former if left unchecked, so you'll probably be helping out in the process.
I recall that there is a general recommendation to have a max limit on the amount of filter feeders in a diet because they have an above average in toxins and heavy metals. There have been suggestions to use such animals to explicitly clear out runoff from terrestrial farms and industries, but then the toxic levels would be so high that the meat is no longer be suitable for human consumption.
That said, they are a delicious source of food that is superior to other meat in many aspect, including being very ecology friendly and space efficient. Anyone that is trying to minimize the damage that a diet has on the environment should consider adding them.
In France, where there are a lot of oyster and mussel farms along the Atlantic coast (apparently, modern oyster farming was invented there in the mid 19th century) warnings and restrictions are regularly issued because of pollution from natural or land farming-related causes, or mollusc diseases.
Shellfish farming is very efficient and ecological, in addition to having cultural value as a traditional local activity, but it does have drawbacks.
I'm having a hard time finding official advice to avoid shellfish that doesn't advise avoiding fish too[1]. In fact, the FDA lists scallops, clams, shrimp, and oysters as having the lowest amount of mercury[2].
It looks like some shellfish toxicity is due to cyanobacteria[3], which is mostly caused by eutrophication from farm runoff. But scaling this system should help with that.
The GreenWave folks seems to have a decent alternative. They base their aquaculture on filter feeders (oysters, mussels, scallops, clams) and autotrophs (ocean kelp, a salad green which btw is the source of omega-3 in fish), with the result that their farms clean up fertilizer runoff from terrestrial farms and reduce eutrophication. Their protein requirements are even lower than fish farms, requiring 0/7ths of what a cow needs (ie no inputs).
Here's an overview from the founder Bren Smith, who incidentally is pretty funny: https://www.youtube.com/watch?v=VjFPdTnclUA&t=4m20s (skip ahead to 8m for just the farm description)