I am ruling out nuclear propulsion because I assume The Pirate Bay do not have access to a supply of fissionable materials. Otherwise, it's Game Over for the MPAA.
Maybe The Pirate Bay should get in contact with North Korea, they'd be more than happy to help undermine the capitalist system from the inside.
Seriously, though, I think this entire thing is merely a PR stunt by the The Pirate Bay. They were never even thinking about doing that. Just like a company they need PR to continue to exist and get donations. Remember, TPB also "tried to buy" Sea Land at some point in time, an oil platform that some people claim is a country? Anyway, it is a good PR stunt, many startups can learn from that.
Sealand was a WW2 anti-aircraft platform. It's too close to shore for doing anything really criminal (like, copyright infringement - murder would probably be fine).
The problem these days is that organisations who don't want to be under US rule have to possess a reasonable number of operational nuclear warheads... It's not enough to set sail and stay in international waters.
Maybe The Pirate Bay should get in contact with North Korea, they'd be more than happy to help undermine the capitalist system from the inside.
Seriously, though, I think this entire thing is merely a PR stunt by the The Pirate Bay. They were never even thinking about doing that. Just like a company they need PR to continue to exist and get donations. Remember, TPB also "tried to buy" Sea Land at some point in time, an oil platform that some people claim is a country? Anyway, it is a good PR stunt, many startups can learn from that.