Concrete hulled pontoon boats are the cheapest to make. No?
Yes. Concrete is the cheapest and most resilient material against salt water. Most seasteaders choose that as their material. I am fairly certain that The Seasteading Institute plans on creating huge 50 meters platforms out of concrete.
Concrete hulled pontoon boats would be good if they are separate from each other. But concrete is fragile when it bangs against concrete over and over. It would be very difficult to attach them together for a large platform. Anything that would be flexible and strong enough to separate the units would likely degrade in salt water over time.
Steel has been used for ship building for decades. It can also bang against each other over and over without degradation. Also, polyurea adheres best to steel, which is what I would use for coating to protect from salt water.
I address the reason for steel in my design outline:
https://discuss.seasteading.org/t/my-viva-vivas-seastead-design/921/2I, also, would prefer steel over concrete. But the expense of steel would be prohibitive right now, at the beginning.
I say start small, and use concrete. Gradually convert to steel as people come on board and add their funds.
Of course, if funds aren't a problem, do the steel right from the start.
Set it up this way, with the idea of conversion in mind, so that it is built in a way that can be converted more easily.