As for meeting singles in their mid to late 20s on this ship, I would say your chances are better than average. Check out the singles section for ideas on where to go and meet other singles.
As for the cruisers, most tend to be United States or Canadian citizens. There will also be a handful of Mexican citizens on board. The crew tends to be international with Southeast Asian and Eastern European natinoalities represented most often. However, crew members including officers can be and are fired on the spot for entering a passenger's cabin without authorization, and approval must be given by the staff captain before you can visit an officer in their quarters.
I have copied the following directly from Carnival's web site about travel dcouments you will need.
What are the requirements for proof of citizenship?
U.S. Immigration determines the correct documents which guests must have in their possession in order to travel. Proper travel documentation is required throughout the cruise and is the responsibility of the guest. Any guest traveling without proper documentation will not be allowed to board the vessel, and no refund of cruise fare will be issued. Documents must be presented at time of embarkation.
United States and Canadian citizens must provide proof of citizenship in the form of a passport (valid or expired for less than 10 years prior), original birth certificate, state issued certified copy of a birth certificate, U.S. Military ID, or original naturalization papers. In addition, guests that are 16 years or older must provide a photo ID. U.S. citizens needing to order their birth certificate may do so by visiting http://www.vitalchek.com/
, an online vital records service.
Resident Aliens of the United States need to have their Alien Resident card AND passport (valid or expired). Aliens eligible to apply for admission under the Visa Waiver Pilot Program, must have a valid passport.
Aliens (non-citizens) are required to carry a valid passport and a valid U.S. Multiple Re-entry Visa. Guests will need to contact the consulates of the islands/countries they will be visiting, to inquire about necessary travel documentation. Immigrations determines the proper documents which guests must have available in order to sail.
For cruises that visit or transit in Canada, citizens of some countries may require a Visa. Guests must check with the Canadian Consulate for documentation requirements. This includes cruises to Canada (from New York) and Alaska cruises.
Please be advised that any guest WITHOUT proper documentation will be denied boarding without benefit of a refund. There are NO EXCEPTIONS.
Hope this helps.