Okay, I have a planet with a global ocean of liquid water, and there are these cities in the ocean, floating a few hundred to a few thousand meters beneath the waves. They have spaceports that allow spaceships to access them directly (good thing that the heroine who visits that planet brought an amphibious ship). Weights and counterweights keep them floating at the right depth, energy is gathered via hydroelectric pumps, the materials used to make them are waterproof, and they have farms for food, and large pots are used to heat saltwater up to evaporate it.
And pumps are used to prevent flooding, with most species living there being amphibious at the very least.
The question is: How do we expand a city like that to adapt to an ever-growing population? We can’t let it rise too high, or else it might capsize or something, or if we’re growing seaweed on the outside, it’ll probably dry out and die when exposed to the air for far too long, nor can we let it sink too low, or else there won’t be enough light for the gardens, and if we go lower still, then the city implodes. And we can’t let it grow too crowded, of course.