Floating cities?

First question: why build cities in the sea? If we consider that cities occupies around 3% of Earth's surface, what would be a reasonable excuse to give up on land's surface? Climate change and the melting of ice-floes.
Considering that, some attempts to design floating structures that would be sustainable cities connected to others and to the environment and being -sometimes - self-sufficient were made during the last few years. The architect Vincent Callebaut desigend Lilypad, Peter Thiel envisioned series of conceptual floating cities, ATDesign has recently designed the Floating City, among others.
Second - and more relevant - question: is that the best way of dealing with the problem we have created?

Sources: Vincent Callebaut Architectures, LILYPAD, A FLOATING ECOPOLIS FOR CLIMATE REFUGEES, <http://vincent.callebaut.org/page1-img-lilypad.html>, accessed 10 June 2014;

ATDesign, ATDesignoffice release design for the Floating City, <http://www.atdesignoffice.com/news/>, accessed 10 June 2014;

Mike Jaccarino, Silicon Valley legend, PayPal founder Peter Thiel envisions floating cities off coast of California, <http://www.nydailynews.com/news/national/thiel-envisions-floating-cities-coast-california-article-1.951630> accessed 10 June 2014.