I'm writing a sci-fi story which includes what I'm naming "buoyant cities". They're cities that can house about 150,000 people, with fully functioning electricity, plumbing, waste management, police force, fire department, and medical services. This city isn't really all that practical, but more a show of the country's capability to construct such cities and form a plan for remaking flooded coastal cities. I've come up with two designs that I think could work, but I'm awful at physics so they may look completely nuts.
This depicts an enormous single body for the city, where the gray would be the region of buildings and other infrastructure. The orange on the outline would work as a buoy, where it could be filled with regular air to float or maybe lighter elements like helium. I'm not sure if this would be able to support the entire city (I highly doubt it) but it was the first design and thought I should mention it.
This second design is a trio of hexagonal plates that are separated by little gaps in between. This one would have the white as the buoy with helium (or hydrogen if there's a safe design) and the gray regions being the actual city. This way the city is broken up and can be held up easier in the case that the previous one would just sink. I'm more confident in this but still unsure.
I'm asking to see if either of these designs are plausible, and if there are any other realistic alternatives that I couldn't find. You can probably tell that I have no clue how physics and buoyancy works so sorry if both of these designs are absolute insane and wasted your time. <3