How would this not devolve into yet another nation-state? The first thing you'll need is an army to defend it. Aside from obvious environmental necessity, I don't really see what's so revolutionary about recreating countries on the water.
The current approach is a phased approach instead of the whole Waterworld thing.
Phase 1:
Set up a prototype in a Special Economic Zone in a friendly nation's protected lagoon. Work on some of the engineering challenges and other things that might come up but in a protected manner.
Phase 2:
Move 12nm out to sea. This has more wave activity which needs to be factored in but gives relative freedom from most of a nation's laws (like when you're on a cruise ship and you have to wait about 30 minutes until they're 12nm away from land before they open up the casino). You are still protected by the host nation and have easy access to the resources on land.
Phase 3:
200nm out to sea. This is where the engineering has to have everything taken care of for large waves and sea activity, but you are outside of any nation's jurisdiction and can be a sovereign nation.
The Seasteading Institute and their spinoff company, Blue Frontiers, are actively in talks with French Polynesia to implement a Special Economic Zone with French Polynesia. I went down there in May and had dinner at the president's house where he gave a speech welcoming us and telling us he really wants to have seasteading flourish in French Polynesia.
