Harvesting algae is very expensive. Especially getting the algae out of the water, and the oil out of the algae requires an expensive centrifuge or substances like hexane.
The statement above needs clarification and qualification. Extracting the last available few percent of oil may require chemical solvents. The majority of the oil can be retrieved simply by pressing it, and the remaining 'cake' can be used for other things- directly as animal fodder, as compost materials, as bio-digester input stock for making methane bio-gas, or dried and burned. The question for a seasteader is not always about efficiency, but sometimes just about effectiveness.
Remote, off-grid locations like seasteads often can benefit from multi-purpose usage more than from complete extraction of a single resource.