Similarly, people seem to make lots of assumptions when they refer to "full-scale nanotechnology." This phrase seems to elicit images of fridge sized home appliances that, when plugged in and stocked with a few "toner cartridges", makes anything a CAD system can describe, and so quickly and cheaply that only the most price-sensitive folks would consider making stuff any other way. It seems people learned too much from the PC case, thinking everything must become personal and local. (Note computing is now getting less local.) But there is no general law of increasingly local production.
I see this a lot in the self-sufficiency tech communities which seasteading tends to overlap with. People seem to implicitly assume that producing power, water, and food locally is better and that technology will inexorably move things in that direction. But as Robin points out, there is no general law which states that local production will be better. It depends on various costs in the particular industry with particular technologies.
I can certainly see the romantic appeal to locality. I yearn for political self-sufficiency, and dependence on large centralized infrastructure is one of the major obstacles to achieving it. If the amenities of civilization could efficiently be provided at the scale of a family, then I would happily hop into a boat and sail for freedom, hoping that others would join me to provide the last ingredient (other people). But as it is, sailboat-sized generation of energy, food, and water, not to mention tools, toys, and computers, is vastly inefficient. My standard of living would drop enormously.
Since locality appears to be very beneficial to political freedom and independence, I certainly hope that utilities get more local, and I applaud technological advances that move us in that direction. But this hope should not be confused with a belief that future technologies will be more local than present ones. Perhaps solar panels will be the energy technology for seasteads, and each family will generate their own power. Or perhaps OTEC will be far cheaper, and so only groups of 10,000 or more seasteaders will be able to get cheap power. The former would make our movement easier, but sadly the universe is not organized so as to make things easy for us.