There is another niche if you make them several miles long and width. Then can use more conventional construction materials like a concrete shell. (We are of course not ready for that)
In that case they should have those giant propellers that seem to spin at a few rotations per minute
We should first build the floating cities. Some old ships welded together surrounded by styrofoam in nets with mud on top and Mangroves, lots of Mangroves... Neptune's park.
An airship made from concrete? Better off using Led for Zeppelins.