For the period 2000-2008, melting Greenland ice raised sea levels by an average of about 0.46mm per year.

If you multiply these numbers up it puts us well beyond the IPCC estimates for 2100 
Professor Roger Barry

Since 2006, that has increased to 0.75mm per year.
"Since 2000, there's clearly been an accelerating loss of mass [from the ice sheet]," said lead researcher Michiel van den Broeke from Utrecht University in the Netherlands.

But in reality, the colonisation of the sea never got much further than this: the occasional undersea habitat where divers stayed for a few days or weeks at most.  Today, there are only two operating in the entire world, but in the '60s and '70s there were more than seventy habitats operated by the US, Germany, France, the USSR, Canada, Poland, and Great Britain.

One, the Sealab programme shown here, was a major effort by the U. S. Navy, which the general public and many of those working on it thought was an effort to rival the Moon landings.  But the Sealab programme didn't fare much better than the Apollo programme.  After a fatal accident in 1969, the third and last of the Sealabs was closed down.  Years later, after the end of the Cold War, it was learned  that the Navy had little or no interest in colonising the seabed.  For them, Sealab was really a way to train their divers for a much more important task: deep diving to tap Soviet telephone cables off the coast of Kamchatka to intercept secret communications.

Yet why did enthusiasm for undersea habitats die out?  Part of the reason was that working on the habitats taught people many things.  One of these is that the sea is not a damp Plymouth Rock; it is a dangerous place where work is hard and slow, where you had to live with everything being constantly soggy,  where the exotic gas mixtures made you sound like Donald Duck, and where the pressure made it impossible to smoke, drink alcohol or soda, eat spicy foods, or even enjoy a boiled egg.  In the end, it became clear that the sea is not a place for settlement, it is an alien world to be cautiously visited.

Sky City 1000 is a possible future urban supertall skyscraper project aimed at helping put an end to major congestion and lack of greenspace in the Tokyo, Japan metropolitan area.

The plan consists of a building 1,000 metres (3,281 ft) tall and 400 m (1,312 ft) wide at the base, and a total floor area of 8 km2 (3.1 sq mi).[2] The design, proposed in 1989 by Takenaka Corporation, would house between 35,000[2][3] and 36,000[4] full-time residents, as well as 100,000 workers. It comprises 14 concave dish-shaped "Space Plateaus" stacked one upon the other. The interior of the plateaus would contain greenspace, and on the edges, on the sides of the building, would be the apartments. Also included in the building would be offices, commercial facilities, schools, theatres, and other modern amenities.[2]

here is two possibilities that I can see(this is inspired by sci fi, that much I admit). One is a cloud city(akin to coruscant in Star Wars). We have had plenty of experience building upwards and having a city in the skies(with our very own flying cars to transport us around) is viable.

I see another option?? underground.. which mean under the surface of ocean... yes... when everything is submerge in water, and we might still live underground..........

again... no matter where we go, the question of resources...

1) O2
2) sunlight?

or can be like microbs.... harvesting energy through chemical reaction instead of sun 

Though, I am concerned about the reactions towards lack of sunlight. I for one dislike being cooped up for so long with no exposure to the sun. Now how would we combat said problem given that we were to build underground/underwater living quarters.

Though, I am concerned about the reactions towards lack of sunlight. I for one dislike being cooped up for so long with no exposure to the sun. Now how would we combat said problem given that we were to build underground/underwater living quarters.

Floating cities would be a more viable option before technology allows us to live in air.....

Living in both underwater and and at altitudes posses pressure problem and various other problems like energy resources...

Generating resources are hardly the matter. The problem lies with humans coping with the change in surroundings. Try sitting in the dark for a week and see how you react to sunlight after said week.

Try sitting in the dark for a week and see how you react to sunlight after said week.

yes.. this is the problem that I think og.. the side effect of total dependent on artificial habitat.. artificial light etc...

what would you feel if you live in city and the only source is from ceiling light, wind from fan, air from ventilation? instead of FRESH AIR? naturally?

I think every city folks would need that.... after hours of work in office...

for example, from my experience.. recently, after wholeday facing PC, and I feel bit "sick" as in had enough of it and nausea and need some fresh air... so I went outswide my hosue for fresh air.. then I observe the trees and then look at my house... there I realise this.. the tree, flower etc... makes me feel alive.. while looking at my house.. even its nicely decorated and with colourful paints, it is dead

that is this called medically? any doc?

don`t have to go to that level...yet

just try to live your days without HP, PC, internet and watches... for few days.... 

see what do you feel

In my opinion,floating cities(on water,not air) are indeed much easier to realize then floating or underwater cities.Cloud cities would need a constant source of energy.Not to mention the hazards of such cities.One wrong calculation and you're done.

Humans aren't fish and we aren't supposed to live underwater.Complications,maybe even death could occur unless humans could construct a total replica of the outside world.

Floating cities would probably be the best choice right now,considering our current technology.Even untrained people could live on top if something catastrophic like 2012 happens.Provided that you don't have sea sickness

Basically its like living on a huge ship,right?
I don't see the need for giant floating cities to move around a lot.
Maybe they would just anchor down at a safe spot and use smaller ships to transport necessities.

Sky cities would pose a problem or two.
As I mentioned earlier,one miscalculation and your city is gone.Not to mention the technology and source of energy needed to maintain it.

Like in the movie 2012(I trust you've seen it),humans built monstrous arks instead of choosing airplanes and what-nots.Almost the same as deciding which city is more suitable to build and live on.