I'm going to travel on a closed-loop cruise, and from what I've researched, I will only be needing a Government issued ID and a birth certificate. Everyone says that it needs to be a certified copy, but on a government site, it says:
"Closed Loop" Cruises: U.S. citizens who board a cruise ship at a port within the United States, travel only within the Western Hemisphere, and return to the same U.S. port on the same ship may present a government issued photo identification, along with proof of citizenship (an original or copy of his or her birth certificate, a Consular report of Birth Abroad, or a Certificate of Naturalization). Please be aware that you may still be required to present a passport to enter the foreign countries your cruise ship is visiting. Check with your cruise line to ensure you have the appropriate documents.
Not really specifying whether or not you need a certified copy. So has anyone had any experience with just a non-certified copy of their birth certificate?
You don't reach customs until after your cruise so it's a moot point.
You need to have the documents required to board the ship so I'd read what the individual cruise line requires. Many times the cruise line requirements are 'tougher' than customs. i.e. health, documentation, pregnancy, etc.