What are the pros and cons and scientific hurdles to building cities on the ocean and why aren't we doing it?
We already have fishing ships, cargo ships, cruise ships, nuclear subs and aircraft carriers. Put a dozen of them all in one place and we get an instant city.

pros:
plenty of space to build
immune to earthquakes
rearrangeable
can eat fish

cons:
no fresh water, but we can put desalination plants everywhere
no agriculture, but we can put a layer of dirt everywhere and grow seaweed