I love the idea, but heavily question feasibility.

My guess is in 50 or so years, it will appear in a place like London, but entirely as a status symbol // art project and not feasible for large style underwater cities.

It's a combination of safety and price.  When you state the water levels are rising, the majority of the people immediately affected by it are too poor to attempt large scale underwater living, and are far more likely to live up on the surface in floating housing instead.  

To the people that can afford it, there are simply far safer places to live (and they can afford to live there).  Though we might be technically capable of doing it on a day to day living style, a disaster (earthquake?  tsunami is in the ocean?) presents a nearly 100% loss of life scenario.  Maintenance and the the constant damage to the structure would be a horribly high upkeep...at least until our knowledge of construction advances considerably.

On the water or underground seems far more likely to me