First, I agree with Will Wilkinson that a seasteading community would likely evolve back to non-libertarian political visions.
Second and more fundamentally, I am for the seasteading idea. There are today many oil derricks, owned and run by energyl companies. There are many cruise ships, with more or less autonomous legal governance. More and bigger cruise ships would be better and if some of them moved more slowly that would be fine too. But when I step on to a cruise ship (well, actually that's the sort of thing I don't do; personally I hate cruise ships), I don't feel I am moving from an inferior political order to a superior political order.
I've wondered whether I should retire on to a cruise ship of the future, but I'm not attracted per se by the "politics" I would get there. I would expect more freedom in the Lockean sense but less of the positive freedom that comes from living in a larger, more diverse, and yes also a more stupid society. I wouldn't live on the Mensa cruise ship either. I'll take some of the stupidity of modern society (the landlubbing version) to get the diversity and the greater number of open niche spaces and free possibilities.
On a smaller scale, I live under different kinds of corporate, non-profit and university governance all the time. That's great, but I don't view their totalized extension as my preferred utopian path.
I'd like people to be smarter, more thoughtful, more tolerant, and more loving of liberty, yet in ways which do not drain away the diversity of the United States, which I feel is the best available foundation to build upon. No matter how good a seasteading charter may sound, any given venture just can't be that credible until it has succeeded for a very long time. History and precedent matter and by the way have you checked in on Estonia lately?
Addendum: Here is Alex on seasteading.