The first step in the Deep Sentinel program, one of the most ambitious ocean-engineering projects in decades, will be completed this year. This initial step, called Vanguard, will shelter three aquanaut-scientists at depths of 100 meters. Eventually, the project will create a modular underwater habitat system that will let scientists live underwater for weeks at a time.
Ocean is tough. Tougher than floating cities. Which are more realistic as real habitable environment.
I vote for Stanford torus stations in various L points.
Or, naturally, Mars.
Before that, of course, there are plenty of locations on Earth hard to live in, but not as hard as underwater domes. They should try that.