Micro nuclear plants, like this one from Toshiba: Toshiba 10MW nuclear plant
Pebble Bed Reactor, safer, simpler and more efficient type of fission reactor. At least one prototype currently operating.
Although nuclear power has a lot of advantages, we need to keep in mind that Seastead will have to be near existing countries, at least in the beginning. Countries probably don't mind a cruise ship kind of operation in their vincinity, but both governments and the public will mind a vessel carrying "scary stuff" and take appropriate action. Although nuclear options may be very cheap and eco-friendly, it is probably better suited for spaceships instead of humble seasteads.
There are many more ways to get energy, and Seastead is an extremely good place to experiment with alternative energy sources like algae since "land" can be made easy. To get things going, focussing on technology that is easy to obtain and to use is probably the best and only way to go.
-I don´t think anyone is seriously suggesting that we fire up a nuclear reactor in the San Fransisco Bay or close to any other countries. I strongly think it´s a realistic alternative for full size seasteads on the high seas however. After all, there are nuclear powerplants all over the world today, even in large vessels. It´s a proven technology that compares favorably against pretty much anything else. The biggest obstacle is political, and seeing as the whole point of seasteading is to get rid of political obstacles I´d say nuclear power fits like a glove. By the way this kind of discussion probably belongs on the talk page. -vtoldude