Before boarding the ship, during the check-in you will have to provide your travel document (ID or Passport).
The general rule is that your travel document must be compatible with the legislation of the visited countries in accordance with your citizenship. For instance if you are an EU citizen traveling within EU countries, you may travel only with your ID but if you travel to EU + Russia (the Baltics), you will need your passport.
IMPORTANT! your travel document must be valid for at least 6 more months from the cruise departure day.