Stealing a container on the high seas is damn near impossible, doable but a challenge.
The middle ground would be taking under tow floating containers that fall off of ships in bad weather - but I daresay that comes under salvage.
The Barbary pirates enslaved about 1.25 million people between 1530 and 1780 (and it was a massive threat in the Middle Ages and ancient times we just don't really have reliable figures).
It was such a massive threat that coastal villages all but disappeared throughout much of Mediterranean Europe. The barbary pirates went as far as Ireland, England or even Iceland to murder an enslave people.
This was an issue up until the 1800s (the first foreign war the US fought was against Morocco and Tripolitania) when the European powers finally took a break from fighting each other to do something about their common enemies who were terrorizing their subjects for centuries.
https://www.imdb.com/title/tt1535109/
Interpret that how you want ;)