The shipowner is invariably located in a PO Box in a jurisdiction which makes Swiss banks seem downright extrovert; however, this is just a shell company controlled by an opaque holding company, &c.
Global shipping is a rather murky business, designed to facilitate tax and responsibility avoidance.
If this sounds interesting, investigative journalist Ian Urbina published a most informative (and scary) book on the subject last year - Outlaw Ocean.
On a related, but lighter note, the memoirs of Max Hardberger (Marine repo man, more or less) is an excellent collection of sea stories of the "Improvised ingenuity and brazenness while under fire" kind. I believe it was called "Seized!"
And slavery, you say? How does this manifest? Are we talking about Somalian pirates conscripting poor people in third world countries or something? I have read that following the collapse of Libya after Qaddafi's overthrow, things degenerated so badly that slaves began to be bought and sold and are even sold there today - but I suppose I've never dug too deep into who's buying them and for what purpose.
And I love a good memoir, especially of a seafarer! That'll be going straight on my Amazon wishlist, thanks!
This is often 'legal' in the sense that the (more often than not illiterate) crew have signed contracts stating that they need to repay a sign-on fee equating years worth of wages.