There are low hundreds of attempts and a few dozen successful ship hijackings per year. It is, overall, a very small part of freight theft, the vast majority of which happens on land. This should be obvious: on land, you don't need a boat big enough to float a container to steal a container. You just need a access to a semi-tractor and maybe a trailer.

If you make the technical demands on a successful hijacking higher, I expect these numbers to fall.

I also expect self-driving trucks to sharply cut theft, rather than enable it.