The woman, a student on a school trip with her class, was allegedly attacked on Saturday after boarding the ship in Civitavecchia.
The young men allegedly raped her after luring her into a cabin with an excuse.
The woman, who is in a state of shock, reported the attack to the captain, who called the police.
The ship was detained in the port of Genoa for initial investigations, including an analysis of video-surveillance cameras.
ANSA