You can leave whenever you want. For an internship you might need to stay to get full credit for your school, but as just a job you can really stop working whenever. The average on my ship was 8-12 weeks.
Answered - Deckhand (Former Employee) - New Orleans, LA
It is possible to leave earlier
Answered - Server (Former Employee) - St. Louis, MO
Typically yes, but you also have the option to choose 12 weeks.
Answered - Assistant Hotel Manager (Former Employee) - Guilford, CT