Outer flat ring can be used as a dock for small boats, or can be absent if it makes the structure cheaper or stronger. Doors at water level with ramps allow structures (floating platforms & small boats) to be deployed and recovered easily.
Spherical Seastead with 1 water-level door & top hatch
Saucer Seastead with 4 water-level doors
Floatable platform to act as bridge, dock, or land. Other types of ballast could be used instead of cylinders shown.