The main benefit is jurisdictional arbitrage - you get to pick whatever flag imposes the least restrictions on whatever use is most important to you. Unlike on land, on a ship if your assigned "country" becomes too oppressive you are free to change it without changing where you live/work. Since flagging countries get paid a small amount (and gain a small amount of status) by virtue of their flagged vessels and have no physical territorial claim over them, flagging countries have more incentive to be generally agreeable than do territorial governments.

So simply "sailing under a flag" gets you quite a lot of the potential benefit of seasteading. Sailing without a flag is something to think about in the far distant future but not really worth worrying about for now.

There are oodles of business models that could make use of this today. Right now, the only one that's really been tried is "day gambling cruise" - sail out of a port where gambling is illegal, gamble once outside the relevant imaginary lines, sail back into port. But there's lots of other business-space to explore, like medical cruises.

Currently many Americans fly to Germany to get medical operations that aren't yet approved here or fly to India to get medical care that is far less expensive than here. Those who live near a port city might save money and time accomplishing the same end - cheaper, more flexible care - by shuttling out to an offshore floating hospital.