The arguments here and elsewhere are all pretty much spot on. Lots of people like the 'concept' of being able to start fresh but few really internalize the challenges. One person I know feels it would be easier to privately colonize the Moon than to build a viable seastead.
That being said, there has been some interesting work in seawater chemistry that suggests you might precipitate out of seawater a form of concrete [1]. Of course it would be hard to precipitate out a floating island but one might imagine building a 'reef equivalent' by growing it from the seafloor up. I personally don't think it is practical but I find the exploratory engineering entertaining and thought provoking.
That being said, there has been some interesting work in seawater chemistry that suggests you might precipitate out of seawater a form of concrete [1]. Of course it would be hard to precipitate out a floating island but one might imagine building a 'reef equivalent' by growing it from the seafloor up. I personally don't think it is practical but I find the exploratory engineering entertaining and thought provoking.
[1] http://www.build.new-atlantis.org/seacrete.htm