Get on a ship without a ticket: stowaway.
What does it mean?
What if the ship is a nuclear submarine? You can't have the ticket for a war ship
To "smuggle" anything is to hide it from the authorities. You can smuggle jewelry through customs without paying duty on it, or smuggle guns or drugs across the border, or smuggle a bag of caramels into weight-loss camp.
So, to smuggle oneself aboard a ship means to get yourself there secretly, whether it's to avoid paying for a ticket, to flee the country, or even to photograph nuclear secrets.
[native speaker & writer, not a teacher]