The Seastead is a standard tile improvement in Civilization VI: Gathering Storm. It is the only Improvement in the game which can be built on ocean tiles, as well as on Coast and Lake tiles, and cannot be placed next to one another.
One of the ultimate expressions of human technology, the Seastead is the only (or just the first of many?) man-made structure capable of withstanding the force of ocean waves. Although it doesn't become available until the final stages of the game, it can vastly improve the yields of water tiles around coastal and island cities. Its Adjacency bonuses work not only on itself, but also on nearby Fishing Boats; what's more, it can be built on tiles otherwise 'inaccessable'. It, along with the Offshore Wind Farm, allows for major development of all Coastal cities
Finally, the Seastead is also good for "reclaiming" tiles that have become submerged due to sea levels rising as the world's climate changes.
Seasteads can go particularly well with a civilization that has picked the God of the Sea pantheon.
By custom, the sea is a place of its own laws, under the control of no nation. This makes it attractive for people who wish to experiment with new models of government, and so the idea of “seasteading” is the creation of new micro-nations at sea. As of the publication of this game, no permanent, sustainable seastead has been created, but many organizations around the world continue to explore the practical implementation of seasteading, whether by creating artificial islands or converting large sea-going vessels (like retired cruise ships) into permanent seasteads.