Notionally, a seastead can be used as a permanent residence which is completely outside of any country's control, allowing for them to be (what would amount to) a "floating haven" where only international laws apply. A boat still has to be registered to a country and, if I remember correctly, has to comply with all the laws under the flag which it flies. That's the primary difference, from what I know. However, the rule of force applies more than ever in international waters so if someone with a standing navy (or pirates) want to roll up on you, you're scuffed unless you can actively contest their actions.

It's an interesting concept, but they definitely needed to make it further away. Trying to just skirt along the edge of a country's territorial boundaries isn't wise, as this couple has apparently very quickly discovered.