The ship has to be under some countries flag, it's not like international waters are a totally lawless zone where you can loot, murder and rape without consequence.

Sadly, being able to "shop" for a county with minimal safety, inspection, and enforcement standards is a valued feature of the modern ship registration system: https://en.wikipedia.org/wiki/Flag_of_convenience

(And, so long as you keep your rape and murder within your ship - that is also a feature. If you are a crime victim on a cruise ship, and the cruise company feels that looking into what happened to you is not in their own business interest... OTOH, you or your survivors may be able to sue the cruise company for damages in a shore-based court. Google for the obvious if your want to see stories about that.)