We went to Italy in a big ship.
We went to Italy on a big ship.
Use on with planes, buses, trains and ships: ‘on a plane’, ‘on the bus’, ‘on a train’, ‘on a ship’.
Use in with cars and taxis: ‘in a car’, ‘in a taxi’.
We say ‘get on a ship’ and ‘get off a ship’, but ‘get in a car’ and ‘get out of a car’.