I was just reading a 1960’s essay by the late Arthur C. Clark-in which he predicted that mankind would be farming the seas by 2000-there would be underwater cities, we would be mining the ocean floors, and land-based agriculture would be obsolete.
Needless to say, none of this happened-yet most of Clark’s arguments were based upon sound science.
Why didn’t it happen?
Nobody can make money off it, although we do drill for oil and natural gas underneath the ocean floor.
I think Clarke, while very forward-looking, and certainly very well-educated, understimated the hostility of the environment. The seas are messy, rough, harsh, and notoriously bad for machinery.
I had the opportunity to mess around inside the hull of a Canadian Navy Destroyer, the Yukon, shortly before it was sunk (story in itself!) as part of an artificial reef project. Much of it was, of course, in very good shape. But parts were corroded all to hades!
Since then, of course, the wreck has become grown-over with weeds and mollusks and other gunk, and corrosion has accelerated. Any actual working underwater habitat would need constant (expensive) preventive maintenance, and periodic massive overhauling.
Even outer space is (slightly) less hostile as a working environment.
I can’t deny it would be fun to spend a day or two in an undersea city and look at all the marine life swimming by, but otherwise it hardly strikes me as Utopia. I sure wouldn’t want to live down there.
You mentioned “farming the sea.” Well, we are doing that. See Wikipedia on mariculture. And someone mentioned deep sea oil and gas platforms.