Tax havens usually are only attractive if you dont have to live there (think mailbox-type of tax havens),
There are also those tax-havens which, due to low taxes, become a very interesting place for investors and entrepreneurs, like Hong-Kong for example. Maybe tax-haven is not the appropriate word in English. I mean "country with very low taxes".
There is also the example of Monaco, where millionaires go to live in order to avoid certain taxes. Their business outside of Monaco keep being taxed, but their personal income and wealth not that much.
If you are a rich Canadian living in Canada, you can say all you want that you dont have to pay taxes in Sealand and that your company is registered there... the Canadian judge wont be very convinced by that argument, though. You will have to actually move there, so the Canadian authorities dont enforce their tax laws. And that wont work with any concept of micro-nation i ever heard of.
The idea of seasteading is that people would move to live there. And by the way, it's not meant to be "for rich only". The idea is to create prosperity for all. Obviously that in the beginning poor people will not be able to afford floating houses, but as times goes by and technology advances, the "ocean tax" becomes smaller, while government taxes quite the contrary...
The only reason that existing tax-havens are still there is that they are actually nations, you know, with armies;
What armies do places like Monaco, Lichtenstein, Singapore and others have?
Floating micro-tax-haven will simply get a visit by the US customs and border protection agency (and/or IRS), who will round up their citizens and haul them off to a grand jury.
Maybe, maybe not. I think it's something worth trying. Way easier to achieve more freedom through this path than through elections, anyway.
An alternative to avoid this danger, at least in the beginning while these floating-nations are not internationally recognized, is to use a convenience flag. This limits your freedom but not that much, since there are small governments out there who make this "convenience-flag" business an important source of revenue. Once the city is big enough, it may abandon the convenience flag and declare sovereignty.
Take the example of the Women on Waves boat. It has already been forbid to dock on ports of conservative nations, but it was never military attacked.