
I first heard about this guy, Reishee Sowa, about ten years ago on some TV news show. He built a floating island out of empty plastic bottles and was able to live on it as it floated near the Yucatan Peninsula.
Unfortunately, the island was destroyed by hurricane in 2005 and since then Sowa has been working on the replacement, named Spiral Island.
The island uses a quarter of a billion soda bottles tied together, with plywood sheets mounted on top of them, then sand is poured over the boards. Mangrove trees, which thrive in salt water, help anchor the sand to the bottles and provide shade.
Sowa is nearly completely independent on his island, gathering drinking water from rain, and using solar ovens for cooking.
More here, a blog devoted to islands
This idea has fascinated me for 10 years, and is increasingly relevant as people try to find ways of controlling the enormous and growing amount of trash floating in the Pacific Ocean.