From the literature, it seems that watermakers are expected to run a ‘cycle’, filling a freshwater tank on the boat, and potentially going some time before being used again. This is consistent with relatively spacious tanks compared to the conservative onboard usage. Any seastead with industrial, agricultural, or nonconserving personal use will have a much higher requirement, either needing more cycles than is expected per person with standard marine units, or significant overcapacity in tank, watermaker, or both.
Is there any thinking here whether reliability figures suggest a larger, commercial-class watermaker, or multiple smaller ones? Either way, given industrial or agricultural uses, requirements will be steady, and suggest a larger than normal resevoir to accomodate downtime, in any case.
It doesn’t make sense on a smaller seastead, but the efficiencies suggest that on a larger seastead, some thought be given to a solar desalination plant, if a significant portion of the energy to run a watermaker comes from solar, which seems likely.
Including a water ionizer with the desal would produce both ionized alkaline water which has many health benefits for drinking and acidic water which is needed for hydroponics and is useful for bathing. A commercially available unit could be reverse engineered, modified, and scaled up to incorporate both desal and ionization in one system.
Solar stills are simple to build but they do take up valuable space. An inflatable structure could be used, but it is hard to beat glass for trapping infra-red radiation. There is also a simple design for a solar ethanol still from the 70′s similar to the one in the link below except that it uses black fabric inside instead of a reservoir inside. The solution to be distilled is trickled on the fabric with the condensate collected in a trough at the foot of the front glass.
A solar concentrator can also be used to boil water either directly in a boiler tube or indirectly from oil heated in the collector tube via a heat exchanger. After the steam is used for power generation, it is condensed into fresh water. It may be possible at some point to extract raw materials from the concentrated brine by running it through a series of zeolite or carbon nanotube filters. Carbon nanotube filters already made for water purification have shown much lower than expected resistance to water flow.
There’s been a lot of work done on solar pre-heating for home hot water production. A pre-heater with a fueled or electrical boiler added should see some rather large efficiency gains, but perhaps could save on some of the space requirements for pure solar. It also would mean that poor weather doesn’t eliminate your water production, just makes it a bit more exensive.
Another point: It has been well-demonstrated that pure distilled water is niehter necessary nor particularly healthful. So water supplies can be augmented with seawater. I have been doing a bit of reading on saline agriculture, and I think you would only need to dilute your seawater for many purposes. Determining which purposes and how much needs more research, before you can propoerly budget your resources.