In situ oceanic construction
Current practice for industry is to hire the cheapest construction site that fits for their needed ocean-going structures (oil platforms, floating cranes, extremely large structural elements of transboarding systems, etc.) and then ship them to their production site.
This practice makes sense in the current world where those structures cannot be built directly on the spot where they will be used, but it puts a number of additional constraints on those structures' design, costs a great lot to ship, and is risky.
Seasteading might open a whole new market in naval construction at sea, directly. A moveable, floating semi-submersible construction dock coming with all the workforce and storage space required for naval building could save on the delivery constraints, cost and time, giving seasteaders a comparative advantage.