Do you expect the city to be under normal surface pressure, or under ambient pressure? This makes a huge difference in design requirements. If the former, then unmodified humans can live in it and can travel to and from surface at any time, but the structure must be enormously strong, built to unbelievable tolerances, and very small structural damage could cause the entire city to implode. Also, humans can only venture outside in pressurized vessels, such as rigid deep-sea "suits".
Will such travel not be enormously expensive for the same reason: pressure? Building the ships to carry them back and forth is just a miniature version of the engineering nightmare of building the city. I suspect that once you're down there you probably have to stay there. But I'd guess only the wealthy would be able to afford to live in such an expensive home, and they'd be the last people to want to do so.
Stephen Hawking claims the world, by A.D. 2600, will be entirely overpopulated. . . .
Then he's a little behind the curve on that. Based upon the trends they see in the birthrates around the globe, many sociologists are predicting that the population will peak at just around ten billion by the end of this century... and then start to drop back. Prosperity is the most effective contraceptive and prosperity is spreading slowly but steadily.
The birthrate among native-born Americans is on the verge of dropping below replacement level as it already has in Europe. Soon the U.S. will be dependent on immigration to support Social Security and the rest of the economy, like Europe. And of course by the second or third generation (depending on the group) those immigrants all become Americans with American habits including our low birthrate.
So whatever conventional figures are being utilized for determining rate of population growth (not sure the rate), the US is not in any immediate danger (Alaska, the Southwest, blah blah blah). Granted, Africa is scheduled to overpopulate in not much past fifty years...but I'm sure we'll be substantially and legitimately colonizing the Sahara WELL before the Pacific.
What's the definition of "overpopulate"? Food production is not an issue in a global economy that is not dysfunctional. The underpopulated, sustainably farmed Western Hemisphere can feed the whole planet, all the way up to that peak population with ten zeroes.
For an entire continent to reach the uniform population density of northern New Jersey would be remarkable but it would hardly be a crisis
per se since people live quite happily and prosperously in northern New Jersey. I've even seen fat people up there so there can't be much of a food shortage.
It's been pointed out that the single greatest factor in determining whether a person is destined to grow up in poverty is
the country he lives in. So the single most effective tactic in reducing global poverty is to encourage emigration.
Honestly, I personally would forego aquatic civilizations for lunar counterparts.
Somehow I suspect that the expense would be equally daunting. Lifting materials (not to mention people) out of Earth's gravity well uses up a prodigious amount of energy. No one's proven to my satisfaction that the infrastructure of a lunar colony can be built from lunar materials, so we could be talking about shipping billions of tons of material a couple of hundred thousand miles. (You could not afford union drivers.
) Then there are the totally unknown medical and psychological effects of living in low gravity. And talk about an expat community for whom a trip home is prohibitively expensive! Nonetheless there are probably a lot more people who would like to live on the moon than in Davy Jones' Locker.