A company called Oceanix is building a prototype floating island as an experimental solution for crowded coastal cities threatened by climate change, the company told the United Nations habitat program Wednesday. Such buoyant islands would be linked together into floating, self-sustaining cities that rise with sea levels and are built to withstand hurricanes, according to a group of architects, engineers and developers who met at the U.N. headquarters. The prototype will be a small-scale version that could be ready within months, said Marc Collins Chen, an entrepreneur and former French Polynesian politician who founded Oceanix.