It seems to me that the perfect location for the next arcology would be the Great Pacific Garbage Patch, also known as the Pacific Trash Vortex.
Numerous architects have experimented with the idea of a city-sized ship (or an artificial, ship-like island), although the floating city in as massive a form as envisioned by science fiction writers and visionary architects such as Soleri, Eugene Tsui, Jean Philippe Zoppini, and others has yet to be designed beyond the art concept stage. From Artect.net:
Despite increasing stresses on our existing societal structure and a world population rapidly growing to unsustainable levels, land based arcologies appear unlikely in the foreseeable future. The answer may reside with the remaining seventy percent of the Earth’s surface, our oceans. Ocean arcologies could utilize ocean thermal energy conversion (OTEC) or derive energy from the ocean’s perpetual motion to provide abundant pollution-free energy. Fresh water would be available via desalination. Despite the inherent dangers and high expense, the surface of Earth’s oceans are already littered with surprising structures from Sea Forts in the Thames Estuary to the famed remnants of Florida’s Stiltsville and of course the ubiquitous oil rig.A few problems exist with dotting the oceans with cities like this, however, one of which is a lack of resources. Undersea mining, of course, is one way to deal with a lack of solid raw resources, but garbage pickup is another. And there's plenty of garbage out there. So, a city floating in its midst could harvest that resource, and thereby improve the health of the surrounding fisheries and the also the beaches of the world.
Sustainability is a big issue with cities like this, so renewable energy and urban agriculture--not to mention fresh water collection or distillation--are factors to consider. Vincent Callebaut is another architect looking at the feasibility of the oceangoing urban structure, as a response to climate change. The illustrations of his Lilypad city are beautiful and very interesting. Worth a look! He also has a concept for another type of floating island/cities.