Look at real-world micronations to get started.
A couple months ago I wrote a scene/vignette where they have a mostly artificial island and politics. Big industries (such as cruise lines) will go to great lengths to prevent any precedent from being set, so regular countries leave them alone. The key is aligning your claim so it would affect big companies if those premises were challenged.
More practically, anyone who suggests that some specific country has jurisdiction will be chased by lawyers saying that one or more others should, instead, so it’s not theirs. … Kings and presidents don’t rule the world: lawyers do!
See: Maritime Law: Murky Jurisdiction
In 2006, a woman onboard a Royal Caribbean cruise ship sailing the Mexican Riviera reported being raped in her stateroom. She immediately returned to Los Angeles, where two FBI agents took her statement a week later, and told her there was nothing they could do.
See: Offshore oil rigs are out of U.S. government’s reach
Big Oil, for sure, will keep things tied up in knots if there’s any hint of new laws or a precedent that would give clear jurisdiction for some government to impose regulations and take actions on their rigs.
“What Country is This, Anyway?”
quoted with (my own) permission.
It was the summer in which she had moved out of the kids’ bunk room and into her own place. It was just a bedroom and tiny bathroom, adjacent to the main house, but hidden away a little so it didn’t exactly feel like next door. She covered the walls with drawings and images torn from magazines or printed out from web sites, making the space her own. She also had a large map of the Bahamas and Caribbean; an old nautical chart from the looks of it. They were near the right edge about two thirds of the way down, among the Lesser Antilles. But their little home wasn’t even on the map! It was probably too old.
“What country are we in?” 15-year-old Beth asked her mother.
“Well… nobody’s sure exactly. And we go to some effort to keep it that way.”
“How can that happen?”
“History, same as everything else. You’ve studied the history of the region, right? You recall how different islands, or even parts of the same island, were claimed by different European governments; and that until fairly recently they switched around or were even fought over sometimes.”
“A hundred years ago, the ground we’re standing on did not exist. If you were on a boat, you’d see the Worthless Duck Rock nearby. That’s all there was — a rock that looked like a duck. Nobody wanted it for anything. Even pirates didn’t like it because there was no place to bury treasure.” Liz smiled in a way to show that this part was a bit of a joke and not literal. “Unless you count the bird poop which was piled up as deep as your waist!”
Beth involuntarily imagined herself standing up to her waist in a pool of bird poop. “EEwwwwww!”
“Even the birds didn’t want it! They would land because they saw something that wasn’t ocean, but there was nothing to eat and no place to build a nest, so they would immediately take off again. And they habitually lighten their load when they take off, so it was just their poop stop.”
“Now England and France both had historical claims, but neither country had it under any specific administrative organization; it was just ignored.”
“Less than a hundred years ago, a rich tycoon named Plait bought the place by paying both England and France. It was just a token amount really, and then they could stop worrying about which of them owned it or if they would ever fight over it.”
“Still, neither country has that rock listed as being part of any region (a region in England and France is like a Provence). Each country divested the land completely, rather than showing it still as a part of themselves but with someone listed to pay taxes. Know what I mean? Normally in such cases the implication is that it is transferred to the government of the person who bought it. But Plait was English! When England did the paperwork, it slipped through the cracks, as they say.”
“That’s just for the original rock that was the only land that existed at the time. All of this,” she waved her arms to indicate everything, “was built. So it’s like a moored ship or an oil platform: maritime law would apply. And it’s, as they say, ‘hopelessly convoluted’. Cruise ships that are outside of anyone’s national waters have been in the news from time to time when no jurisdiction will do anything about a reported crime. The cruise ships in particular want to keep independent from the country where it’s from, so they can have gambling, alcohol, and whatnot. There should be a ‘port of registry’ that the owners get to choose; but from those news stories we can see that it doesn’t always help.”
“So look at the ownership. We have U.S. citizenship so that should do, right? Nope! The real ownership is by holding companies which are wholly owned by family members who live in different countries. The holding company that directly owns this resort is in Amsterdam. But this property, and the others on this artificial cay, is leased from the owner of the ground itself. We have a large interest in that company, which your Daddy bought into in order to fix up the place; have the town where staff can live, electricity that always works, and so on. But that company itself is incorporated in Switzerland.”
Beth’s head was spinning. “So no country has it on its books officially, and anyone who tried to figure out who ought to have it will be confused.”
“More practically, anyone who suggests that some specific country has jurisdiction will be chased by lawyers saying that one or more others should, instead, so it’s not theirs. And not just our lawyers; cruise lines and shipping companies have a huge interest in making sure that legal precedent is not established.”
“I get it, but what is it? What’s it called? Where are we, if you needed to name that state of affairs?” Beth loved words, and it would bother her if some nebulous concept didn’t have a name. Liz knew that about her daughter, and knew what she needed.
“I might not be getting this exactly right; you can look it up. But we’re a de-facto non-secessionist sovereign micronation. I’m basically the queen.” Liz takes a mock bow. “As long as we don’t harbor fugitives or otherwise get others upset with us in a major way, cooperate with the neighbors, and stay under the radar, we’re an independent state. If anyone does try to muscle in, it will create international incidents out of proportion to what they’re trying for. That is, for example, if Guadeloupe tries to annex us, France will be starting a war with the UK. Kings and presidents don’t rule the world: lawyers do!”
“Also, recognition isn’t an on/off thing like a light switch. It’s more like a dimmer switch knob. Different governments, agencies within a single government, corporations, and NGOs can independently recognize us or not. And the extended family over-all — not just Taft branch but distant relatives; the fortune goes back over 150 years to the Industrial Revolution — are close-knit with many important NGOs. Having organizations like Red Cross and Médecins Sans Frontières recognize us a a neutral place and having an open invite to use the grounds if a neutral location is needed, is not as good as recognition by the U.N. but it’s pretty close.”
Beth had had no idea her little home, Lizard’s Hideaway, was actually a player on the world stage. Yet it wasn’t on the map!
Since your story is an advanced tech-based utopia, the idea of building up filler rock or building a structure about 30 meters from the shallow water up to the water line should not be a problem, or beyond your budget. Don’t forget, it happens over a period of years and you can keep adding to it!
You could even use biotechnology, such as growing it out of coral.
What I’m saying is: go with artificial construction at sea. Then “getting land for your own country” becomes a political issue, not a technological problem.
My own choice checks a lot of your boxes: contiguous, nice climate, sea access, isolated. But the planet is huge and you could find a sea-mount (something that’s not quite an island) somewhere else or make it a permanent anchored floating raft.