Pangea Castle isn't even that safe, Sabo broke into it pretty easily, however keeping it on a ship that's always sailing would be practically impossible to find, I mean, Zou is essentially impossible to find since characters need a Vivre Card to get to it, and it's an entire island, imagine a single ship, on top of that the Burned Man can create whirlpools to keep away any ships that get close by chance, it's actually pretty much the best way of hiding one.