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.
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.