Food. What everyone else around the world eats - imported from wherever it is produced.
Through various methods of trade, food produced on land could be purchased for use by seasteaders.
Vertical Farming could provide local food and/or a source of income.
It could also be possible to build houses with a greenhouse on top. This makes the house(s) independent from the main land for food. I think this is the best place to build a greenhouse because it keeps the outskirts of the island free for adding another unit.
The ocean itself is a plentiful source of food which already supplies much of the world with valuable nutrients. Through the use of nets, fishing lines, or other methods wild fish can be harvested for use by seasteaders. It has also been suggested that a seastead could construct underwater enclosures in order to create mobile fisheries, making it much easier to control the production and harvesting of fish, without fear of overfishing local resources or needing to migrate solely for the purpose of hunting for fish.
A major concern that many people have is the contamination of fish by mercury or, more specifically, methyl mercury. In the United States, mercury pollution is regulated by the Environmental Protection Agency (EPA) and the Food and Drug Administration (FDA). Mercury is a deadly neurotoxin that kills nerve cells, causes blurry vision, lack of coordination, slurred speech, and even death at high levels. Children exposed to high levels of the compound pre-natally can suffer slowed development, blindness, cerebral palsy, and other birth defects. For years, however, scientists have debated the effect of low levels of mercury exposure and at what levels they can begin to impair human health. Some studies have shown that exposure levels up to 10-20 times the EPA recommended minimum seem to have no major impact on human health.
Here is a link to a study by the University of Rochester titled Exposure and Fish, which discusses the problem and the findings of their study on low level mercury contamination in humans.
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