The big problem with most futuristic ocean-based designs is that the designers often go for spectacular architecture but impractical infrastructure. And, just as on ships, the greatest problem is power, I.e. energy generation.

One reason we don't have any on Earth now is that such urbanities would require massive amounts of energy, well beyond the capabilities of nuclear power plant tech.

One design consideration I was looking at a few years ago was to generate power via Massive Local Current Array generators which work just like standard turbines, but are much larger.

The real-world problem of constructing in international waters is a legal one. I suppose on a water world this isn't an issue.

Greg C