I?ve been thinking along these lines of moving airports to sea for a few years. The urban land is so valuable, and the noise pollution and congestion so much, that shifting this all to sea has to be a strong economic case in most big coastal cities. The method I would propose is to float the airports on large sacks of fresh water. Fresh water floats on salt water, with 2.5% of volume above the surface. Polymer bags of fresh water, up to several cubic kilometres in size, could provide a very big stable and safe platform for an airport or other floating island. Large cylindrical fresh water tubes under the ocean could also provide buoyancy for a submarine ocean road or rail line inside the tube.
I?ve developed related ideas further with a proposal for an Algae City in the Gulf of Mexico. See
http://www.bautforum.com/attachments/science-technology/11097d1256609680-patent-reverse-global-warming-algae-city-gulf-mexico.jpg 
