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
As an odd side note to this...Stephen Hawking made a comment about space colonization being the route to save humanity...the faster we get to the stars, the faster humanity gets to a state where it can't be eliminated. This is based on a simple principle I like to refer to as human redundancy...we are currently 100% dependant on Earth as a species and an apocalyptic event on earth could end our species. If we were to setlle on...say a moon on Jupiter...something hugely traumatic could end life on Earth but still have Humanity as a species survive. Oceanic colonization isn't this degree of redundancy, though it does relocate some of us underwater, it does very little from a 'survival of humanity' standpoint. Not sure if thats really an arguement against oceanic colonization, but it does show we have other area's of colonization more suited to the 'survival of species' line of thought.