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While the island in the book/movie is clearly imaginary, similar floating vegetative islands can and do exist.
As explained on Could the floating islands of "Life of Pi" really exist?, some exist in various parts of the world that have vegetation and sometimes even animals (dancing deer on an Indian island).
Indeed, I've been to the floating Uros islands in Lake Titica, Peru, which are, however, man-made, but have people living on them.
But an acidic, carnivorous island with teeth in the fruit of the plants it bears? Nope :)