Sails are an interesting propulsion option. They could be deployed in the space below the platform and above the waves, with the spar itself acting as a mast. A keel would of course be necessary, perhaps by making the submerged flotation oblong in shape. Because water is much denser than air, it takes a high ratio of sail area to wetted area to propel a boat. Seasteads have a lot of wetted area, so they'd need a lot of sail. Large sails are quite expensive, movement would be slow, and a square-rigged seastead would be unable to head much into the wind. The fact that the wind is a powerful sustainable energy source may turn out to compensate for these disadvantages, or it may prove to be an impractical option.