Cruise ships are, in effect, floating gated communities. All crew is required to carry photo identification at all times. All passengers are also photographed and provided with identification before boarding. You must keep this ID with you at all times! You will need to present it in order to leave or board the ship. You may also be required to pass items through a detector device, both when leaving and boarding the ship.
The Coast Guard, ship and local authorities are extremely vigilant where cruise ships are concerned. Your safety is truly their #1 priority.