After some research i feel that Fuel Cell cogeneration would be a very good way of powering a seastead in its everyday energy usage. I dont believe ive seen this mentioned in the book already.
That is, unless you are aiming for energy independence, but i do not see that as a realistic goal, or if it were realistic, even as a desirable complicating factor. Seasteading wil be difficult enough as is.
There are practical units entering the market for domestic use at the moment. Natural gas seems most prominent now, but methanol would be possible in the future. Hydrogen ofcource aswell, but it isnt practical in storage and transport.
What these things do is reliably and efficiently generate electricity from these fuels, and make the waste heat available for heating and hot water. Efficiencies are >30% electrical already, which is competitive with centralized power generation. And that figure is likely to go up considerably in the future. A few advantages over conventional generators are that they do not require maintenance and their are silent and more efficient. A disadvantage is that they are less compact than combustion type generators, but thats not a problem in a stationary application like a seastead. The initial investment is still higher at the moment, but they are already starting to become an interested alternative without subsidy right now, and i think that will definitely hold true in a few years time.
Another advantage is that they are modular, and are well suited to work together with intermittent energy sources, such as solar panels.