A couple of guards with weapons?
A caretaker, maybe, or maybe someone with the skills to go manual at need. Weapons are a separate problem - many ports forbid them aboard. Hence some of the problems off Somalia. I could see a human skeleton crew as a backup, and anyway the Seafarers' and Maritime Officer unions would definitely be putting the pressure on the ship owners. That's just another hoop, IMHO, but it's there.