A widely respected yacht designer (Gary Mull, who died of cancer in July 1994 at the age of 55, http://members.dca.net/pwink/ranger/garymull.htm) had theories about a floating island, particularly, Alcatraz. His research into it was initiated by his curiosity of the area's wind and water cones, and the odd currents in and around Alcatraz in the bay, and why sailboats had such a difficult time negotiating it. From the following website comes this quote: "It was in the great storm of 1772 that the Spanish discovered that the premonitory on which they had built their fort was not attached to the mainland, but actually seemed to be what is called now in geological circles as a floating island...they had built their fort on what apparently was a large pumice plug, blown loose at some time by a volcano..." The article goes on to cite the usual government secrets, etc., and is an intriguing short read. Don't you get an interesting image in your mind when you think about a huge plug blowing off the top of a volcano and landing in the ocean? Sploosh! I'd like to give credit to the Lost blogger who posted this theory and link on TLC (I think) last year, but I can't remember who it was, I'm sorry.
In 2005 someone devised a way to put some of Central Park on a barge and drag it around the Hudson River as some sort of natural installation art where, "Seeing a green island moving up and down the waters is supposed to make you think about your own physical location." Ho-hum. I have rarely liked that kind of art...if it's not downright ugly, it seems interesting until you read the pretentious description of it that is supposed to shake up all your preconceived notions about your life. Moving right along.
Jules Verne seems to have written a book called "The Floating Island" (also going by other titles) but I can't seem to find too much info on it. It's mostly about a man-made structure though, not a natural island.