Once our goals, motivations, and obstacles are understood, we can examine designs for meeting them. We'll cover a wide variety of structures for living on the ocean, from boats to oil rigs to undersea habitats. We'll also examine some of the basic design choices which must be made. These include whether a seastead should be free floating or fixed in one place, whether to use breakwaters or pillars to stop the waves, how to make floating-cities modular, and whether to purchase new or used structures. With these considerations in mind, we'll present more detail on our preferred design, the spar platform. This structure avoids the massive energy of ocean waves by keeping its platform above them and its flotation below. In between is a thin pillar which presents little cross-sectional area to the waves.