[Ren is correct. Law is a hugely grumpy goth. That's why he's always got this dumb hat on the keep his eyes shadowed.]

People travel by boat; Big Blue is mostly islands. From my experience, cultures do tend to spread between islands in the four seas fairly easily, but the currents in the Grand Line make leisure travel difficult and so the countries there are far more self contained.
[He pauses, and then makes a face as if he'd been tricked into explaining so much about his home world.]