No ballast. I worked on this quite a while and started a thread below on it.
Most ocean going vessels are roughly 1/3 draft (below the water line) and 2/3 freeboard (above the water line.) Ballast helps optimize this ratio. There are notable exceptions. The largest cruise ships have only about 20% draft, with 80% of the vessel above water. The Allure of the Seas also spends almost all it’s time in protected waters like the Gulf of Mexico, venturing onto the Atlantic only during calm summer months.
More draft helps fluid dynamics for a ship in motion. This means nothing for a city. There is no advantage to draft for a floating city.
More freeboard means the city will react more strongly to wind, but also will ride above waves better. The whole need for a breakwater is moderated if most of the water motion passes under the city.
Your picture example shows a canal type houseboat with about a 12" draft. That’s the same as my floating house design pictured above. A 40 ton structure on a 60x60 platform only drafts about 12 inches.
Stability is gained by keeping your wind dynamics clean and in the case of my design, by using marine gyroscopes to stabilize the structure.
I finally concluded that for a floating city, the best draft possible is the least draft.
There is some value to having “solid” feeling walls, so 2lb per square foot EPS on a 6" thick wall would be nice. You don’t want to be able to punch your fist through it, because people would thing it is flimsy. Also concrete on the outside for durability and to keep the fishies from eating the foam. Aside from that, the lighter you are, the more you are free from the subsurface forces of currents and waves.