Fish Aggregation Device
The ocean has many variations, just like land. Nutrients and sea life are not equally distributed. Deep water far from shore has less density of macro-sized sea life than near-shore. Reefs and other hard substrate attract and provide habitat for more fish than open water and sandy bottoms.
Fish tend to gather around floating objects. This may be to hide in the shade, to eat algae and other biological material from the object, or to hunt other smaller fish tat are doing so. This provides some opportunity for a seastead to enhance their local eco-system.