What would there be to stop the mercenaries from simply taking over the "country" they were hired to protect? It's not like this hasn't happened before....

Off the top of my head, the only things of value on a seastead are the salvageable materials, which would be a hassle to collect and sell, and the net worth of the various residents. They have no strategic value, it would cost a lot to keep them supplied, and you're in the middle of the ocean. To me, it would be much easier to to just bill them 10 mil a month for protection against a low-risk to non-existent threat until they can't afford it anymore.

"To me, it would be much easier to to just bill them 10 mil a month for protection against a low-risk to non-existent threat"

The protection racket business model?

"Nice seastead you have there, real shame if it was to have a revolution".

Wouldn't even have to go that far. Seasteaders are already paranoid enough that you can just play on the fears they already have.

"Nice seastead you have here. It'd be a real shame if some country decided to come out and take your strategically worthless, resource barren, unstable platform from you."

Then sit back and collect a paycheck.