Most seasteaders would prefer to define their own sovereignty. And of course this has its issues with respect to being attacked by various folks, pirates, navies of various countries, etc. A fairly good writeup appeared in Ars Technica [1]. James Grimmalman, a professor of Law at New York Law School, did a legal analysis which was fairly widely cited as well.
[1] http://arstechnica.com/features/2008/06/seasteading-engineer...
[2] http://stevereads.com/papers_to_read/sealand_havenco_and_the...
You try that. If you don't fly the flag - you are not under the protection of any sovereign nation and are essentially subject to the powers of any other.
> territorial waters of a country where such activities are illegal.
No - they don't get harassed until they get detected - if we knew where they were - we'd take them out in international waters.
> anyone can opt into and opt out of at any time.
You can do that now - move to Somalia.
> risks and decide if the reward is good enough
So just ignore that part then - caveat emptor right?