It seems quite likely that our large sea-cities will be free-floating, for various reasons. First, there are not very many seamounts to anchor to in international waters (> 200nm from any place a rock sticks its nose above water). Second, the need for an exit from unhappy nation-neighbors and the dictates of dynamic geography to be modular suggest that we’ll get more freedom and safety if we aren’t tied down. Just common sense, really.
One downside to being in the vasty deep is missing out on one of the traditionally neat things about the ocean: its multicolored bounty of fish, ranging from lovely to wonderfully freaky in appearance. But perhaps we can nurture our own artificial reef, hanging below us and carried along with us, in a similar way to how Delaware has been turning old subway cars into fish condominiums:
Sixteen nautical miles from the Indian River Inlet and about 80 feet underwater, a building boom is under way at the Red Bird Reef. One by one, a machine operator has been shoving hundreds of retired New York City subway cars off a barge, continuing the transformation of a barren stretch of ocean floor into a bountiful oasis, carpeted in sea grasses, walled thick with blue mussels and sponges, and teeming with black sea bass and tautog. “They’re basically luxury condominiums for fish,” Jeff Tinsman, artificial reef program manager for the Delaware Department of Natural Resources and Environmental Control, said as one of 48 of the 19-ton retirees from New York City sank toward the 666 already on the ocean floor.
I can imagine a seastead doing the same, perhaps with old cargo containers. The main issue I see is nutrients – many parts of the ocean are nutrient poor. The garden might require an associated pump to bring up nutrient-rich water from the seabed…which is going to be tough if the seabed is miles deep. Or perhaps organic waste from the city above will be a sufficient nutrient supply