So where do I begin. I think that technology is important to what and where a seastead can be placed. How would one build a marina in open water with huge swells and waves, at least a ice dish would create calm water to dock in.
Without something to create calm water than the options for placing a marina are extremely limited to the point that a prick coast guard ship can easily pull up and ruin everyone’s day.
If seawater is frozen from the outside-in like when ice cubes and molds are made than it does trap some minerals in it of which the ocean has many and gives ice interesting properties. It’s not about the salt.
The world I live in nothing is impossible until absolutely proven. Maybe it might take insulated panels set up in a form with tubes run through like rebar is with concrete to create a ice reinforced structure. What matters is the cost to make the insulated panels, refrigeration plant and wave energy device to power it vs other options.
As a final note it doesn’t mean anyone will make anything out of ice but options are always good.