But what if you just avoid such dangerous places? Hurricanes don't happen everywhere. The South Atlantic for example is calmer. The Mediterranean too.
How deep would you need to go to avoid the most dangerous waves a sea like the Mediterranean may hit you with?
Obviously by avoid I don't mean 100% ignore it, but just resist it without having your "house" rolling/turning strongly (or breaking apart!) and without getting seasick.
I would go twice the depth of the longest period wave known to have happened in the region and in water at least 10 times as deep as that period. Anything less and life could get interesting. I remember a storm where at 200 feet down, the low pressure area between the wave crests pulled our submarine up to the surface. It was interesting getting back down. Luckily there were no broken bones and all the gear was repairable.
If your goal is to avoid seasickness, I recommend staying on dry land. IF that is a dominant concern in your design for a seastead, you really should not be going to sea. Not everyone should go to sea.
The supporters of the idea claim that most of the time the ocean is calm enough so that you can leave your submarine afloat. The top of the submarine would be like a balcony. It wouldn't be much different from an apartment after all.
Once in a while you'd have to sink for protection though. I wonder how sudden would that happen. I mean, if you left your home floating and went to work nearby, do you have enough time to come back to it and sink before things get serious?
You would contemplate commuting from your seastead to where??? You would allow the systems you depend on to protect your family and all you possess to be in anything less than perfect working order? Please don't seastead and if you must, please don't do it near me.
That's part of the argument of the seasteading institute (which is against submersible designs).
Those for it claim that such complexity is not necessary in a seastead use case. They say, for example, that you'd never need to sink deeper than what a snorkel can reach, so no need for fancy equipment to get oxygen from the sea water. Most of the military grade equipment available in modern submarines would also not be necessary either, according to them.
A 400 foot snorkel?

That "military grade" equipment is actually SUBSAFE grade. Go read about SUBSAFE and the USS Thresher for a better understanding of why SUBSAFE is not cheap. Those CO2 scrubbers and O2 generators are not military luxuries, they are what allow you to live long enough that you can fix the crap that put you at the bottom of the sea. Besides, what is so fancy about purifying water and splitting it to get O2? If that is not a simple process you can sketch out on a napkin over beers with your neighbor, you might not want to be going to sea in a submarine.
If I remember well, they claimed that you'd be able to have a concrete submarine with available inner space comparable to that of a 60m
2 apartment for something like 200k, what would make it comparable to the costs of a house in the French Riviera for example. Do you agree with this assessment?
The main supporter of this idea has already produced private concrete submarines:
http://concretesubmarine.com/I will not go to sea in a concrete vessel ever again. Please never mention that idea again.
They appear to be worth every dollar that was paid for them.
Because they would not be crippled by regulations and taxation. That's the whole point: the freer the economy, the more potential it has. A seastead based on libertarian principles would likely be freer than any other economy in this world.
How does that freedom from non-libertarian regulations and taxes compare to the increased taxes and regulations that will be part of keeping the seastead floating? We _know_ there will be an increase in cost to do anything at sea. You _hope_ that libertarian regulations and taxes will be more efficient and effective in an at sea environment than _any_ current system anywhere on land and that the increase in efficiency will offset the increase in cost.
Even if you are correct that libertarian regulations and taxes are dramatically better than anything we have today, as soon as Nauru decides to emulate you, your competitive advantage is gone.
Most american nations didn't have a shared heritage or shared language when they started. Same goes for Australia and other 'new world' nations.
The organizing force of most new world nations shared a heritage and language. The individual national histories are how they consolidated the people around them to that heritage and language. That process (and conflict) continues today. Ideally your seastead won't have to deal with a civil war over
debt based labor slavery or "Indian wars" over land use or religious wars between the Minnowites and the Sharkafians. Of course, if you are not careful about making sure you have "common ground" with your neighbors you may find they will take your ground and leave you "treading water".