So I seem to have permanently broken Havana somehow. I was climbing up a wall, when all of a sudden the screen went black (save for some HUD elements), and it sounded like I was hearing every nearby conversation at once.
Every other location works, but when I'm in Havana, everything is black. I've tried fast traveling around the city, and I've also tried sailing into port. Either way, same result.
Through sheer dumb luck, I appear to have assassinated an entire city.