And if the floating sea-stead is a libertarian paradise, everybody's probably armed in which case your cost/benefit odds are pretty bad....
The real threat is less random piracy than sovereign invasion. The good news is that the list of countries with a credible ability to mount a military assault on a seasteading platform is short. The bad news is that it is also basically the exact list of countries that the seasteading effort is trying to get away from. To me, that's the biggest problem with the idea, it is intrinsically based on the sufferance of the very entities you are putatively trying to avoid, and that's basically true regardless of the politics of the platform.
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There have been plenty of cargo companies paying ransom to get their ships back, but those ships were crewed by a handful of people and taken over.
Further, oil companies and cruise lines benefit from the protection of nearby navies as well as deterrence via international law enforcement. Would a seastead be provided the same?
Less than a millions dollars or so of equipment and a few dozen militia would be enough to repel any pirate incursion to date. Pirates are in it for the money, they aren't going to be willing to accept large casualties.
IMO, a natural disaster is a bigger concern for a seastead though.
A few 3" naval guns should handle any pirate incursion. It wouldn't really cost very much to arm a seastead enough to make it a very costly target for non-governmental attackers.
I agree though that they have much bigger issues than pirates.
[1]http://www.nytimes.com/2011/03/24/world/africa/24qaddafi.htm...
[2]http://en.wikipedia.org/wiki/Chiquita_Brands_International#P...
Perhaps a comparison may be drawn to China's ongoing desire to repatriate Taiwan.
Off the top of my mind, I would say the pirate attacks wouldn't be a big deal. Advanced detection and defense systems could be put in place to warn of uninvited vessels approaching. Remember piracy is a big deal precisely in those places least likely to have seasteads (why would someone build a resort, hospital, or tech incubator off the shores of Somalia?).
Natural disaster is of course and issue, but I don't know if the probability of natural disaster would be any higher on a seastead versus on land. Natural disasters frequently aren't natural at all, at least the consequences aren't. Just compare the consequences of the Haiti earthquake versus Fukushima. When there are incentives to protect property, it will be protected. The probability of a disaster is uncontrollable anywhere you live, the extent of devastation is controllable.
> Natural disaster is of course and issue, but I don't know if the probability of natural disaster would be any higher on a seastead versus on land.
It's not the probability of a natural disaster happening that is a problem, it's the impact. One of the main reasons for a seastead from what I've heard of seasteaders is to avoid paying taxes they don't believe in. But it costs money to rebuild these things and/or to have another nation-state evacuate them. My impression, admittedly not based on in-depth research, is that seasteaders are after all the benefits that nation-state can offer without paying the taxes for it. But I'll be happy to be proven wrong because I think the idea is really cool.
Also at the scale you're talking about, small fast boats, a well armed private militia would be just as effective, as a billion dollar destroyer.
A few 3" guns will stop anything a pirate can throw at them.